* DM particle haven't yet been found.

* The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations.

* There is a broad variety of Dark matter models that are consistent with simulations.

* It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO.

* There are a few cases where you can find tensions with the existing DM based paradigm. I.e. the is test pointed in the article involving galactic bar in the paper "Fast galaxy bars continue to challenge standard cosmology". Note the toned down title. In this paper they just compared one specific feature of galactic disks in the simulations to the data and show that it doesn't match. I'd argue there are many reasons that could be the case that doesnt' involve killing DM. The same applies to other tensions.

* A final point. Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background.

Dark matter models are fine-tuned to reproduce certain observations. These don't really count as evidence in favour of DM.

> * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO.

MOND has more points in favour for it as a preferred theory than DM models. See [1].

> Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background.

I'm not sure why I should find your claim that DM is "essential" as persuasive given MOND has received orders of magnitude less attention and less development than DM models. I would certainly not bet any money on MOND being incapable of reproducing these observations.

[1] From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm

That's the definition of physics. Models are fine-tuned to match the observation and be predictive.

That's not what "fine-tuned" means. Epicycles can be fine-tuned to reproduce the motion we see from elliptical orbits. However, we should clearly prefer the theory of elliptical orbits because it reproduces observations without any extra parameters needed to fine-tune it.

One may need to add more free parameters to a theory, but those parameters need not be finely tuned.

A free parameter is one that must be measured or estimated, as opposed to dependent parameters which are fixed by the theory (and possibly its free parameters).

The pointwise energy-density of dark matter, or its contribution to the stress-energy tensor at a point -- a free parameter -- is presently only estimated, but that estimate is not finely-tuned. In fact, that's part of the practical problem of proving particle dark matter: the pointwise density is underdetermined by observation. As a result, we don't know whether the DM particles must all have very small masses, or whether there can be a mix of masses right up to masses comparable to that of Jupiter. We also don't know the density-gradient very precisely within galaxy clusters and individual galaxies. However, we get coarsely-similar galaxies and clusters in simulations for a wide variety of choices in these areas. So therefore, the dark-matter density (and its gradient, if any) are not finely-tuned. We must therefore search the parameter space to exclude many families of possibilities as significant contributors to dark matter energy-density. (Which has happened: many types of MACHO have been ruled out by observation; various supersymmetry candidates for WIMPs have been ruled out by experiment; most "warm" dark matter has been ruled out; and so forth).

If only the distribution of dark matter was finely-tuned, we could quickly narrow down the search space : <https://en.wikipedia.org/wiki/Dark_matter#Composition> (the last box in the table is for the cases where one has the mathematical freedom to choose to move some or all of the non-metric-tensor additional field(s) from the curvature side of the Einstein Field Equations to the matter side (where the stress-energy tensor is found), without prejudice as to whether it is more naturally (in a conceptual sense) a function on the curvature (which is why the cosmological constant appears on that side, where it could just as easily be written down as a function on stress-energy)).

You're right I've been too loose with how I've been using the term "fine-tuning". Adding a parameter is not fine-tuning so much as just tuning.

The point I was trying to make is that both adding parameters to a theory to fit observations that it didn't predict, and fine-tuning it's parameters to fit should count against its plausibility, since they're both forms of tweaking the theory to match observations after the fact rather than making accurate predictions that follow naturally from the model being proposed.

Those characteristics are: no "non-luminous" or "hidden" matter sources -- the glowing, refracting, and obscuring dusts are the sole sources of gravitation -- and in circumstances where post-Newtonian corrections are vanishingly small, strong concordance with Newton and Kepler except at the lowest accelerations.

The orbits of galactic blobs of slowly-moving molecular gas that produce characteristic spectral lines amenable to Doppler redshift study are almost always in those special circumstances, and they have higher accelerations "corewards" in galaxies and low accelerations "outwards" in galaxies. These gas blobs are most interesting at the leading and trailing limbs of edge-on discoid/spiral galaxies, or face-on in large elliptical galaxies with negligible bulk rotation (the Doppler-shifting blobs move in and out, almost entirely radially), precisely where the accelerations are smallest.

The question we then ask is: what if we have a blob or some other spectrally well-defined object in a galaxy is on a very fast orbit? We can no longer ignore post-Newtonian corrections, or we lose the MONDian match with spectral lines. This is what drives §7 ibid.

If we think of the expansion as:

Total gravitation = empty flat background + MONDian matter relation + higher-order terms (h.-o.t.s. or HOTs)

the relativistically moving but still-MONDian matter is the generator of the HOTs, which has been measured (by among other people Pavel Kroupa, the author of the fine article at the top of all this discussion).

Abandoning the MONDian matter relation is easy. Just recast:

Total Einstein gravitation = empty flat background + unknown stress-energy tensor + higher-order terms

and adapt the stress-energy tensor. Nothing in the theory of gravitation has ever suggested strongly the nature of matter which generates the stress-energy, but we can get constraints from other areas of physics like the standard model of particle physics (or even classical electromagnetism, as was the case when General Relativity was new), and we can get candidates from programs seeking to expand or alter the Standard Model to solve open non-gravitational problems. Thus: various types of electrically-neutral particles like axions or supersymmetrical sparticles have been considered because "someone else" (i.e., not a relativist) hypothesized them. (Observational cosmologists can find evidence to constrain such theories, and in practice helped kill off several supersymmetry extensions of the standard model, for example). Low-mass primordial black holes were a candidate too, because there were non-galactic-dynamics reasons to suspect they could exist in significant number. But those black holes (or very dim tiny stars or isolated cold Jupiters) could maybe generate the unknown stress-energy tensor.

Retaining the MONDian approach can be done in a couple of ways. Hold to the idea that the "luminous" stuff is the only source of gravitation, and then ask whether relativistically-moving MONDian matter is coupled to an unexpected background:

Total gravitation = empty flat background + curvature corrections + MONDian matter + HOTs

(where all that's left in HOTs is basically gravitational waves from relativistically-moving MONDian matter)

or alternatively whether there is a further field or fields to which slow-moving MONDian matter couples only very weakly, but fast-moving MONDian matter experiences quite strongly. That means the HOTs imply the existence of one or more new fields.

Are these fields free parameters? Good (and open) question.

What types of fields are allowed? §7 ibid. explores this largely in terms of adding fields to the Einstein Field Equations (EFEs) of general relativity. The fields conceptually modify Einsteinian gravitation, so are written on the curvature side of the EFEs. However, as the authors of the survey note, in most cases for purely mathematical reasons the fields could be written exactly equivalently as a function on the stress-energy tensor (the matter side of the curvature = matter relation).

Why are we thinking in terms of an adaptation of the Einstein Field equation in the first place? Because it works well for length scales much shorter than ~ten kiloparsecs, and might work well for length scales much greater than megaparsecs. In particular, it works so well in the solar system that it cannot be ignored: it is the effective theory of gravitation near us. So however one might eventually write down a relativistic MOND theory, it must be possible to re-write it into a modification of the Einstein Field Equations for use in the solar system and in systems like Hulse-Taylor or triple-pulsar J0337+1715.

Additionally, analysis through tools like the Parameterized post-Newtonian formalism <https://en.wikipedia.org/wiki/Alternatives_to_general_relati...> is extremely useful if one recasts some arbitrarily-different-from-General-Relativity gravitational theory into something for which PPNF variables can be found. The discipline of submitting to PPNF by rewriting a theory for PPNF friendliness often is personally useful for the theorist keenly interested in her or his not-like-General-Relativity theory of gravitation.

One might say, aiming for neutrality, that if one adapts the EFEs in such a way that the adaptation can appear on either side of the EFEs, the "conceptual" weight of the adaptation is perhaps more aesthetic than physical.

However, that's not quite fair: outright adding extra stress-energy dark-matter-style lets one decouple the "adaptation" of the galactic relation from galaxies themselves. One could put particle dark matter in parts of the universe where there are no electrons or protons. For instance, before electroweak decoupling and big bang baryogenesis, or in the extreme distant future where we have basically isolated black holes and relic microwaves (and neutrinos) that are undetectably cold/long-wavelength. Even around "today"'s epoch, dark matter could be put willy nilly well away from clusters of galaxies.

A modified gravity theory, relativized along the lines of Famaey and McGaugh's surveyed options, might also be able to do this. Strange curvature just existing apart from any matter. (One can do this by a choice of a strange background in a post-Newtonian correction formulation of standard General Relativity too.) But to be MONDian I think that you would want attractive gravitation to appear only where there is matter that interacts electromagnetically, because far from such matter (attractive) accelerations will be very low, and the central feature of MOND is that Newtonian gravitation needs adjustment at very low accelerations.

So for a fully-relativistic MONDian theory of gravitation, although maybe mathematically we could treat the low-acceleration as a field that we could drop just about anywhere, doing so would violate MOND's spirit. There is thus no vacuum + MOND field(s) gravitational solution that is remotely physical. This is certainly not the case for particle theories of dark matter. Those basically insist that it is perfectly reasonable to drop dark matter just about anywhere. One can easily expect to simulate a vacuum + cold dark matter solution to the Einstein Field Equations, and that the simulation might accord with some part of our actual universe.

Finally, without further diving into the aesthetics or bets about what particle physics at CERN and the like might discover about mechanisms that generate stress-energy, I think the "fight" between MONDian modified gravitation and Einsteinian gravitation with dark matter is irrelevant to astrophysicists (as opposed to theorists who deal with non-astrophysical systems), even those who work on galaxy dynamics. It is almost certain that it will be possible to find an initial values formulation for either type of "final conceptual" answer to the observed Milgrom relation. It's just a question then of finding out how to populate the initial values surface, and then letting the dynamical laws go to work on those.

(The initial data and laws for each theory (MOND or GR+DM) reformulated this way will necessarily differ, perhaps by a lot, but the approach of evolving a set of initial data will be the same (this is done in many areas of physics having little/nothing to do with gravitation, after all)).

That is, the core of the MOND/particle DM fight might be more about bets on whether the single minimally-coupled metric tensor approach in General Relativity is not always suitable, or whether the Standard Model of particle physics is incomplete in relevant ways, than about whether one is really more suitable for calculations than the other. (The bet is also not strictly either/or!)

I'm honestly not sure who will really care in practice, if it's ever "finally" decided one way or another.

This is the contention I have with the "fine-tune" argument against DM: there's still tuning needed from the MOND side to both make it a predictable tool across the universe and also ensure it still acts like EFE at home. Everybody's tuning/tweaking because no one has anything novel.

To bring in an interesting analogy: it's almost like the debate over functional and OO programming paradigms. MOND seeks to tune the "function" or theory to better fit all observations. DM seeks to tune the "objects" or observations to better fit the theory.

The whole point of MOND is that our gravitational theory is inadequate, and exploring what modifications are needed to match observations is the way move forward, rather than assuming GR is correct and searching for the missing mass it says should be there. We actually already know GR must be wrong because of its singularities, so this insistence on looking for DM despite the constant failures is doubly strange.

MOND's process of exploring what modifications are needed isn't "fine-tuning" in the way that DM has been tuned because MOND is not a final or complete theory, and nobody has presented it as such.

This is not why we "know" GR must be wrong.

You keep making very strong assertions that any expert -- including those not hostile to MOND, or even positively in favour of properly capturing the empirical MOND relation into some other theory (whether that's the standard one or one like Bekenstein's) -- simply would not make.

I do not know what you are trying to do here on HN, but it does not seem to be an attempt to honestly help people who are interested in dark matter but who lack expertise; it doesn't seem to be an attempt to acquire expertise you clearly lack; and it doesn't seem to be an attempt to work out how you yourself might better understand difficult concepts through a process of ELI5/ELI12. It seems, frankly, like you just enjoy being rude on hackernews.

In order to make up for the observations above that might be fairly taken as leading off with insults, I'll supply some related real, standard, uncontroversial physics discussion.

The principal problem with General Relativity is that the stress-energy tensor when generated by real matter (which we know can behave quantum mechanically) must in principle be able to encode superpositions of (at least) spin, momentum, and position. However, the Einstein Field Equations are fully classical, so how to do this encoding is not known. As a result "hacks" like taking the expectation value of the stress-energy T_{\mu\nu} -> <T_{\mu\nu}> are used to make the EFEs work at all. Unfortunately this averaging generally destroys superpositions, so one runs into problems like: if we took a fairly large molecule and put it into a superposition of position in a laboratory full of sensitive precision-stabilized accelerometers, what should we expect them to show? To which the answers are unattractive (and hopefully will be tested soon enough by direct experiment (at e.g. <https://www.npl.washington.edu/eotwash/node/1>)). A little more detail in <https://en.wikipedia.org/wiki/Semiclassical_gravity> or for more advanced readers e.g. <https://journals.aps.org/prd/abstract/10.1103/PhysRevD.47.45...> which corresponds with <https://arxiv.org/abs/gr-qc/9304008>.

This is far far far from the strong gravity regime near gravitational singularities. It's a problem we could test in a spacecraft in deep space, or in a laboratory here on Earth. No black holes required.

Singularities, while annoying for developing Cauchy problems as one likes to do in many areas of physics, are not immediately fatal to the theory for several reasons, including the probable correctness of the cosmic censorship hypothesis and the long term stability of realistic black holes. Indeed, at least General Relativity is a complete theory that makes a prediction about what happens at its highest energy levels (measured in curvature invariants, like Kretschmann's). The Standard Model of Particle Physics is not presently "UV-complete". (More details @ <https://en.wikipedia.org/wiki/Physics_beyond_the_Standard_Mo...>, "The Standard Model is inherently an incomplete theory.") The converse is that while General Relativity is an inherently complete theory, it requires care in encoding the stress-energy tensor, any "background" curvature, boundary conditions, constraints, energy and coordinate conditions, and so forth. It is not an easy theory at all, especially when one does not understand all the mechanisms that generate the stress-energy tensor. Its successes as an effective field theory in astrophysical applications are legion, and completely contradict the remainder of your second paragraph.

All of the above equally applies to relativistic MOND (except the last sentence of the preceding paragraph: relativistic MOND generically gets the acoustic peaks of the CMB temperature power spectrum wrong, and usually badly wrong for the odd-numbered peaks, principally and importantly because of how different MOND is, by design, at assigning pressure in equations-of-state, and how that matters before the formation of the first galaxies). Milgrom's work has afaik always been in the regime where quantum weirdness has been negligible (and indeed, for most of MOND's history was even wholly in the regime where special-relativistic weirdness was negligible).

MOND has been explored for more than fifty years, and Relativistic MOND for more than twenty years, by some of the biggest brains in gravitational physics, including Mordehai Milgrom <https://arxiv.org/abs/1310.3373>. There are endless publications about MOND even today (tiny sample: <https://arxiv.org/search/advanced?advanced=&terms-0-operator...>). Nobody working in galactic astrophysics is ignorant of it, or of the phenomenological Milgrom's law. Most theorists working in gravitation have played with it so they already know:

> exploring what modifications are needed to match observations is the way forward

So I find myself wondering: do you genuinely not know that MOND "is a thing" already (and has been for decades) in the relevant academic and professional circles?

I don't at all dispute that GR is very well confirmed in many regimes, but it's still an indisputable fact is that GR is definitely wrong because of its singularities. Whether it's wrong in other regimes remains to be seen.

Some like to reframe its breakdown as GR being "incomplete", but that's just a cute euphemism for "wrong". This point couldn't be more obvious so I'm not even sure why you would dispute it.

You can of course argue that GR may only be wrong on small scales or very high energies where quantum effects become relevant, but that's just conjecture. A conjecture with good reasons given GR's empirical success so far, but still not established fact.

How GR fails may or may not impact the argument for DM, but we should acknowledge that the fact that GR is wrong actually might impact the justification for DM, which should already make us more skeptical of the core case for DM.

> So I find myself wondering: do you genuinely not know that MOND "is a thing" already (and has been for decades) in the relevant academic and professional circles?

Of course it's a thing. I'm not sure how I can be more clear on what arguing, so I'll try one last time: while I appreciate the interesting details you've put into your replies, nothing you've said changes the fact that DM's highly regarded status in this field is just undeserved, and that MOND still gets less attention than it deserves given its unexpected predictive successes [3,4].

DM was very plausible early on, but it has repeatedly failed to provide clear predictions and failed to match observations without post-hoc adjustments, where MOND has had some very surprising predictions that matched observations in ways it had no right to if DM were true. I don't believe I ever disputed that MOND also required tuning to match some results, so clearly neither theory is adequate to explain all of our observations in their current forms. At the very least we should be able to agree on that.

Other specific points:

TeVeS is one relativistic generalization of MOND, but hardly the only one possible is it? How much attention has been paid to MOND and relativistic generalizations when compared to tweaking DM theories to match results, would you say? Wouldn't you agree that all of the attention to DM, and the dogma that DM must simply exist [1], detracts from exploring other possible avenues, like questioning assumptions that underlie our theories, as in [2]?

On a final point on tone, I have no idea what you mean by me being enjoying being rude. I haven't said a single rude thing that I can see, I've only disagreed with some interpretations of the data and claimed that DM enjoys an undeserved status as preferred model, and that MOND deserves more attention, all positions with ample evidence.

[1] https://www.preposterousuniverse.com/blog/2011/02/26/dark-ma...

[2] https://www.mdpi.com/2075-4434/9/4/76/htm

[3] http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html

[4] http://astroweb.case.edu/ssm/mond/Claim-Rebuttal.pdf

I'm not an expert, but it seems like Dark Matter theories allow the DM distribution to be tuned. Is that the case? Can you say, "oh, there must be a clump of DM over here with mass X". If so, that would give you lots of parameters to tune per situation you're modelling. It may be that IS how the universe works. The problem is that the theory is hard to falsify if it has so much wiggle room to fit every situation.

Well, yes. Next thing is to pick the easiest of these models: Occam's razor. But pondering whether such model is "correct" won't bring any further improvements.

Isn't MOND completely falsified by the discovery of galaxies without dark matter?

As for galaxies without DM, those observations are still being debated, but even if true, that doesn't entail MOND can't explain those results.

You mean the theory that has received 2-3 orders of magnitude less attention than DM has some conceptual holes to patch up? Colour me shocked. The fact that DM still fails despite all of that investment should worry you more.

> Particle DM can explain all of these things at the same time without re-tuning the model every time.

DM has already been tweaked and fine-tuned to patch up its own endless list of holes, many of which were recently reviewed by McGaugh [1]. Many more remain despite that, like the dynamical friction discussed in this article.

[1] https://tritonstation.com/2022/04/08/two-hypotheses/

Finally, I'll just note that literally nothing in Sean's post suggests McGaugh's work is "dubious" in the least, so I have no idea where you got that notion.

Are you actually arguing that DM is the only conceivable theory that can possibly explain those observations? Be serious. This obsession with DM when it's been clearly falsified numerous times is restricting imagination and attention on novel approaches.

Actually you didn't. Perhaps you claimed that in another thread, but I quoted exactly what you wrote in this thread, which is that only DM can explain those results.

> Since Occam's Razor tells us to go with the simplest solution, science goes for DM, instead of DM+MOND.

That's not a correct formulation of Occam's razor. Occam's razor tells us not to posit the existence of more things than are necessary to explain observations. Observations already tell us that DM either does not exist, or doesn't exist in the amounts most people think exists, ie. most DM models have been refuted. Therefore, even if DM exists, something else is still needed to explain our observations.

This is just a plain lie and since you obviously have no interest in further educating yourself in any way, I will now stop wasting my time with replying to you.

As long as a opposing theory can't replicate these simulations from initial state to known "now" state without the use of some form of "whatever" it is quite interesting to call out that there is nothing like "dark matter" (whatever this something may be).

But that is just my layman's take.

Whether it's a "shtick" or not, his basic point seems like one that is worthy of some kind of substantive counter argument, if one exists. His basic point is that small galaxies orbiting larger galaxies should experience Chandrasekhar dynamical friction if dark matter exists, but no such thing is observed. Therefore dark matter cannot exist.

Has any astrophysicist who supports the dark matter hypothesis published a counter argument to this?

Edit: A published paper of Kroupa's that gives much more detail about his point is here:

https://arxiv.org/pdf/1406.4860.pdf

How about therefore thing we've yet to observe if it exists fails to behave as we naively expect.

This is a grand claim from one piece of work and feels more like a straw man grab at further research funding rather than finding the heffalon at the LHC https://arxiv.org/abs/1303.7367

Or in other words Occam's razor says you have a stripey horse in northern Europe rather than a zebra.

As I understand it, the "behavior" of dark matter in Kroupa's analysis is the behavior that is claimed for it by dark matter proponents. So if dark matter doesn't behave the way dark matter proponents say it does, the burden would be on the dark matter proponents to modify their models to say how it does behave.

AFAIK none of these alternatives offer a serious explanation of gravitational lensing for instance (yet, I'm happy for someone to make a theoretical breakthrough that explains away the need for an unknown new particle, but that just opens more questions around CP violation and so on)

Why? Who made you an infallible authority?

> none of these alternatives

The basic point I described is not proposing an alternative to dark matter, and I'm not asking for a refutation of Kroupa's well-known preference for alternative gravity theories. I'm just pointing out that Kroupa has described a prediction of the dark matter hypothesis that does not appear to match observations. I'm wondering if any proponent of the dark matter hypothesis has a counter argument to this: either an explanation of why the dark matter hypothesis doesn't make the prediction Kroupa is describing, or an explanation of why that prediction does match observations after all. This seems to me to be a perfectly reasonable question that does not require any discussion of alternative theories at all.

I'm not infallible but I do have a PhD in the study of CPV in particle physics so I understand this at a level enough to pass a doctoral viva.

Alternate gravity theories struggle with gravitational lensing. This is a proven observable optical phenomena in the study of the universe.

Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity. This is colloquially referred to as Dark Matter. A gravitational (matter like) effect, which doesn't interact optically via standard EM as we can see the galaxies and stars. Hence the name Dark Matter.

Are not what I'm asking about here. I know Kroupa pushes them, but that's not the part of Kroupa's work that I'm asking about.

> Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity.

Not in the specific item I'm asking about. In that specific item, he's not talking about any alternative theories of gravity at all. He's pointing out what he claims is a conflict between the predictions of the dark matter hypothesis and actual observations. I've already explained what a counter argument to that would look like. Do you have one?

I'll try to explain.

Again dark matter is an attempt to explain a collection of observations that don't match current accepted theoretical models.

There "classical dark matter hypothesises" is just that. That these observations can be explained by a quantizable matter candidate of some form with some yet to be determined properties.

The only non particle descriptions of "dark matter phenomena" is a modified gravity framework. These attempt to address the "classical dark matter phenomena" whilst disentangling it from the "CPV phenomena". This is fine but breaks the big bang model at the stage of baryogenesis (matter formation from pure energy).

There is the possibility that you can explain "classical dark matter phenomena" without demanding it also be responsible for CPV. However, if you do that the CPV step of the big bang model demands additional particle candidates which then raises the problem of what do they look like and where are they and do they interact with the Higgs field and therefore have mass.

There are no counter arguments to "classical dark matter phenomena" because they are just that. Observed natural phenomena which do not fit into our current understanding of the universe.

A "complete dark matter theory" tends to explain CPV+DM. These again don't make many testable predictions other than a (potentially TeV scale) weekly interacting particle candidate. So the testable prediction is that a dark matter candidate exists and we can observe it.

> I've already explained what a counter argument to that would look like. Do you have one?

I'll choose to read that in a relaxed manner, albeit it's a bit direct. There is no counter argument to natural phenomena. The main 2 theories are that dark matter particles exist and we can find them or we can't. MOND et al make few predictions and struggle to observe defined "gravitational phenomena", hence why there is little interest in them. There are differences between DM particle models but almost all agree that the major premise is that we might/should be able to see certain particle types given how they interact.

To me this reads as the answer "no" to the question I asked. Nobody actually has a counter argument. "Experts" have simply chosen to dismiss the matter without any argument at all.

To expand on this somewhat: you appear to believe that I am asking for some kind of separation of observations, so that we consider only the particular observations Kroupa refers to (orbits of small satellite galaxies around large galaxies) and ignore all the other observations. That is not what I am asking. I am asking for a simple response from "experts" who support the dark matter hypothesis in general to these two questions:

(1) Is Kroupa correct in his claims about what dark matter models of "halos" around galaxies predict regarding the orbits of small satellite galaxies around larger galaxies? If not, why not?

(2) Is Kroupa correct that actual observations of the orbits of small satellite galaxies around larger galaxies do not match the predictions of dark matter models? If not, why not?

These seem to me like straightforward questions of the kind that scientists pose to other scientists all the time, and it does not seem credible to me that "experts" would not have responses to them along any of several obvious lines:

(A) Kroupa's computations of the predictions of dark matter "halo" models are wrong. Here are the correct predictions, which match observations.

(B) Kroupa's claims about the predictions are correct, but his comparison with the data is wrong. Here is the correct comparison with the data, which shows that the models match observations.

(C) Kroupa's claims that the predictions don't match observations in this particular domain are correct taken in isolation. However, when the entirety of all observations are considered, including ones that have nothing to do with individual galaxies (such as the evolution of the early universe), the dark matter models do much better at minimizing the overall error between model and observation than any of the alternative models, including the ones Kroupa favors.

You seem to be leaning towards something like (C), but if so, you could just say (C) and be done with it. Instead, you appear to be trying to avoid any possible admission that there is any discrepancy between dark matter models and observations at all, which IMO is not a good strategy. No model ever matches 100% of observations.

> dark matter is an attempt to explain a collection of observations that don't match current accepted theoretical models

I would not put it that way. I would put it that dark matter is an attempt to explain a collection of observations using currently accepted theoretical models (General Relativity and the relevant solutions to the Einstein Field Equation), by making use of a free parameter in the models, namely the assumed stress-energy tensor (or distribution of matter, if you like). By adding a "dark matter" component to the distribution of matter, which does not interact electromagnetically and therefore is not visible to us by any means other than its gravitational effects, many observations can be matched much better.

MOND, by contrast, proposes modifying the theory of gravity in general. That, to me, is a much bigger change than adding a component to the distribution of matter.

> There is no counter argument to natural phenomena.

So what? I wasn't asking for a counter argument to "natural phenomena". I was asking for a counter argument to a claim that a particular model makes predictions that don't match observations. Claims like this are made all the time in science, and the appropriate response is to give a counter argument along the lines I described. I have not seen one, and as I noted above, all your remarks have given me the strong impression that there isn't one, and that "experts" have no interest in providing one. That answers my question, although no doubt the inferences I draw from that answer will be very different than yours.

Please take a look at the modified gravity proposal that I have put forward. This idea incorporates a mechanism that would explain why gravity might slightly deviate from Newtonian and/or GR in some circumstances.

This paper also adapts GR in such a way that it is consistent with galactic rotation rates, the anisotropies of the CMB, and cosmological expansion -- while showing that the simple operation of gravity is the cause of each of these phenomena.

This idea, Cyclic Gravity and Cosmology (CGC) predicts that there are discrete specific sizes allowable for macro-objects. The instability of Bennu and the fact that it behaves more like loosely held scree rather than a compact mass -- is an example of a mass that is not exactly at one of the discrete allowable sizes. Please also refer to the link I included wherein I uploaded a video simulation of the formation of a solar system using this type of force law. (This is in section 18 of the paper)

I would greatly appreciate any comments on this idea. A copy may be downloaded here:

https://vixra.org/pdf/2203.0032v3.pdf

(D) Kroupka is making bold claims that have to be verified first in the scientific arena very loudly and publically. That is odd. He is claiming that DM doesn't exist which is effectively saying the particle solution to the problem can't exist. This breaks the big bang model as we understand it as I've stated. And is offering a gravity based solution which explains some small phenomena which he claims that not only that this isn't modelled accurately currently but that it can't be using the DM models. That claim requires huge effort to correctly simulate the matter field as it _might_ exist and that requires more assumptions. This could mean certain particle solutions are ruled out or that we need to revise the particle model (fantastic we learned something), but it's much less likely that it's completely dead just because one test says so.

His model may be valid. His data might be good. His conclusion has little to do with either.

Just because a theory doesn't match all observations doesn't mean people stop working on it. If that happened the foundations leading to the Higgs scalar boson wouldn't have been researched as at the time everyone was interested in understanding other field theories.

Ok some background.

DM research was introduced as a solution to the problem of the distribution of mass within galaxies. I.e. orbital velocities of stars within a single galaxy.

In order to try and understand this phenomena models were used to simulate the distribution of "matter fields" within galaxies. Now modern modelling has evolved to simulate complex systems of multiple galaxies or even larger. We now suspect and have strong evidence for huge mega structures (MPa mega parsecs in size) made of some "DM goop" in the universe.

Modeling of this is notoriously difficult and relies on numerous assumptions because we can't model everything exactly so we take sensible shortcuts which shouldn't impact the physics of interest for the simulation.

Large simulations have problems the further they evolve. This is a stochastic not model dependent statement. Modelling a thing we're yet to observe is even more difficult.

Most claims that DM can't exist are people modifying gravity in some way to remove DM as a physical particle that you can build a DM hammer out of in principle. These claims are not entirely baseless but really struggle when you try to match large gravitational structures we have seen so far in the universe (cosmic filaments).

Kroupa claiming his simulation has to be the only simulation to explain a problem (if he makes or infers this) would be false. Everything in science has requirement of reproducibility which goes beyond share the source and run the tool.

Kroupa claiming that his model perfectly describes this set of observations might be interesting, but is yet to be demonstrated at different scales. Again this has to satisfy orbital momenta of stars within a galaxy as well as demonstrating larger scale effects such as gravitational lensing.

Dark matter simulations _all of them_ will have some amount of disagreement or uncertainty. That is the nature of scientific computer modelling and comparison to scientificly collected data. Uncertainty and error estimation/understanding is a huge part of this field. Anyone claiming exact results better have a huge amount of data to back it up.

This is either a conversation that would have been easier by Skype. I'm guessing you don't have a strong background in a formal science. I'm not saying this from some pedastle, it's a joint question/observation of you focusing on the socialogical implications and exactness of a thing.

There is no lack of interest from experts to talk about their field. There is almost never a lack of wanting to move away from the "status quo" in the scientific community. Especially in blue sky particle and astro research at a professional level. The public perception of this existing comes from 2 issues. 1) "loud" individuals who shout loudly that they're not being heard will be ignored. That isn't discourse, that is someone acting like a small child within this area. Unfortunately they often get awarded by funding and we're stuck with them. As they get pushed aside for shouting rather than discussing they tend to get louder. 2) the media loves to paint the image of scientists sitting there as gatekeepers to some theoretical framework. This is fun in movies etc but mostly we talk about this in science education as a case of remember the mistakes from the last 100 years. As a result you will be looked at strangely for dismissing an idea out of hand without proof or testability.

Meaning basically that other experts haven't had time yet to check his work? How long is that expected to take? The paper I referenced was published in 2015.

> He is claiming that DM doesn't exist

Yes, on the basis of his claim that dark matter models make predictions about the orbits of small satellite galaxies around larger galaxies that don't match observations. As I've said, these kinds of claims get made in science all the time. Scientific models are supposed to be tested against observations. When someone sees a mismatch, they're supposed to say so. That's part of how science is supposed to work.

> This breaks the big bang model as we understand it as I've stated.

More precisely, it "breaks" the belief that we can construct a valid model of our universe using standard General Relativity, on the basis of the claim that we cannot find any assumption for the distribution of matter that gives a satisfactory fit against all observations. That's the larger claim he's making and the basis of his advocacy for alternative theories of gravity, yes. But, as I've already stated, that larger claim is not what I have been asking about.

> it's much less likely that it's completely dead just because one test says so.

That doesn't change the fact that there should be some answer to how that one test comes out: either it does come out the way he claims, or it doesn't. Which is all I was asking about. If the answer is "nobody else has had the time or resources to check as yet", then that's the answer.

> Ok some background.

While this is all valid information it's not information I really needed. I'm well aware of the difficulties involved in constructing these models and checking them against observations.

> I'm guessing you don't have a strong background in a formal science.

Your guess would be wrong. I don't currently do scientific research for a living, but that doesn't mean I don't have a lot of background in how science works.

> you focusing on the socialogical implications and exactness of a thing.

I have no idea where you are getting this from. I haven't asked about "sociological implications" at all. You're the one who keeps bringing that up. And I have already explicitly recognized that no scientific model matches observations with 100% exactness.

> The public perception

Has nothing to do with what I've been asking about. I haven't been asking general questions about how the scientific field of cosmology works or whether or not cosmologists are supporting a "status quo" or whether or not "dissenters" in science get a fair hearing. I've been asking a very specific question which I've already described several times.

Likely never, as I have explained. Someone loud will be pushed aside from the community and simulations of this type/scale take too long to 'jsut test' or 'just repeat'.

> Yes, on the basis of his claim that dark matter models make predictions about the orbits of small satellite galaxies around larger galaxies that don't match observations. As I've said, these kinds of claims get made in science all the time. Scientific models are supposed to be tested against observations. When someone sees a mismatch, they're supposed to say so. That's part of how science is supposed to work.

This is not a reasinable assertion based on his work so, in simple terms, no.

> More precisely, it "breaks" the belief that we can construct a valid model of our universe using standard General Relativity, on the basis of the claim that we cannot find any assumption for the distribution of matter that gives a satisfactory fit against all observations. That's the larger claim he's making and the basis of his advocacy for alternative theories of gravity, yes. But, as I've already stated, that larger claim is not what I have been asking about.

No you cannot make claims in isolation that break the rest of science that's not how it works. Ever.

> While this is all valid information it's not information I really needed. I'm well aware of the difficulties involved in constructing these models and checking them against observations.

This is _essential_ to the discussion to underatand what is being discussed. To throw the topic aside is showing a lack of focussing on the article, but I think

> That doesn't change the fact that there should be some answer to how that one test comes out: either it does come out the way he claims, or it doesn't. Which is all I was asking about. If the answer is "nobody else has had the time or resources to check as yet", then that's the answer.

Again, a loud person jumping up and down with a simple test over a single overvation will not get the community to divert funding, time and effort. Reasonable arguments for diverting of limited resources are to approach the community with fact based evidence and reason.

> Your guess would be wrong. I don't currently do scientific research for a living, but that doesn't mean I don't have a lot of background in how science works.

Please stop being blunt and defensive, I'm trying to be polite.

> I have no idea where you are getting this from. I haven't asked about "sociological implications" at all. You're the one who keeps bringing that up. And I have already explicitly recognized that no scientific model matches observations with 100% exactness.

You brought it up in a paragraph with the theme of "how the community should change" which is the main shouting point from this article.

> Has nothing to do with what I've been asking about.

as above.

OK, to avoid this going further I think I've in good faith discussed everything in reasonable detail. You may feel different, you're entitled to and you're entitled to reply. But to move on I feel I've given you the large benefit of an experienced insight that is for you to make of what you chose. Have a great day/week/year, I won't be replying further.

Ok, that answers my question. I won't comment on the rest except to say that I don't think I have the same views about how science works (or should work) that you do. But scientists all have the freedom to choose what to work on, so ultimately that will determine how all this plays out.

> You brought it up in a paragraph with the theme of "how the community should change"

To be clear, nothing I have posted in this thread was intended to make any such claim. As I have said all along, I was only interested in a specific answer to a specific question, which you have now made clear. As I noted just above, scientists are all free to choose what they will work on, and I have no issue with that at all. Kroupa is free to express his opinions about more general questions like "how the community should change", and other scientists are free to ignore him, which is what seems to be happening. All of that is beyond the scope of what I was asking about.

1: Michelson-Morley experiment: https://physics.stackexchange.com/a/472611

However, "dark matter" is an admittedly lose framework it could be due to 1 new type of particles, 10, or higher dimensional effects and new physics we don't understand yet. Saying "therefore it doesn't exist" is ignoring the mountain of decades old solid data showing gaps in our knowledge exist.

And as for dark energy... "The universal expansion is accelerating". Well done you now know as much as 99% of the physicists in the world on this strange unexplained problem.

In any case competing theories will need to converge towards new experimental evidence and therefore towards each other, and then names and starting principles will be unimportant.

If I'm straw manning you, it is unintentional. How would you propose dark matter be falsified?

Over the next 20 years or so we will likely cover a huge amount of the phase space of direct observation of dark matter like candidates assuming they weakly interact with the standard model.

If we observe it, great we now know a huge chunk more than ever before of the universe.

If we fail to observe it then we have to go away and explain the Dark Matter problem another way. Axions are my personal favourite here because I claim nature for her beauty is sometimes horrible. But this could (in the most scary scenario) potentially be non quantisable interactions between unknown aspects of the universe. At that point we need to further our understanding of potential graviton candidates to evolve our theories of spacetime.

DM isn't a solid prediction like an aether it's a statement of a big gap in our knowledge that has implications to asto and particle physics models.

The big problem is then CP violation (why more matter than not, real why are we here at all stuff). This can't be explained by the standard model and stuff we can see and touch as far as we know. So we assume that this huge amount of it that we can't explain must come from interactions with new particles somewhere in some way at some energy scale.

Physicists try to narrow down that phase space of where and how but a lot of this stuff makes the search for the higgs, look like cheating and watching where your parents hid the Easter eggs. There is scary little to directly go on so theoreticians have to get clever.

Edit: There is also little to say that extra-SM CP violation _must_ be from the same source as Dark Matter astro candidates. The combination of the 2 fields is much more a car of. "Hmm we both have a missing piece in our jigsaw that has 3 of the same sides and a 4th we've never seen. That's odd let's look together to try and find it." Modeling of the early universe to today relies on some unknown amount of CP violation occurring and we know we see it in particle physics which describes everything we see eat and touch. So why the amounts are so different (orders of magnitude different) keeps some scientists up at night.

It's asif Newton observed an extra solar comet and therefore decided the Principia must otherwise all be wrong. It's a big step to say shinny thing in sky makes the Apple not fall as we observe it.

Yet there is no proof that such simplifications or constrains are valid on the scale of a typical galaxy. Moreover, when people managed to use full equations of General Relativity, then it was possible to account the effects previously thought to require the dark matter hypothesis.

So until it is mathematically proven that the current simulations are valid approximations, I will remain very skeptical about the dark matter.

The best-well known effect is the "missing mass" in rotating galaxies. Well recently we found galaxies that look like they have no dark matter in them - i.e. they behave as you would expect from classical gravity/centripetal force arguments [1]. So while you can tweak General Relativity to match some of the effects in typical galaxies, you would then expect the results to hold in all galaxies. The "outliers" are easily explained by Dark Matter - they just don't have it for whatever reason.

The simulations of Universe formation, sure have their limitations, but look much worse without DM.

Galaxy clusters look like they have unexplained mass in them. Crucially, this isn't only from observing their rotational speeds (so like galaxy rotations) but also from looking at the strength of gravitational lensing they induce. This is as compared to fluid-dynamics-y calculations of gas temperature, pressure etc.

Bullet cluster is another interesting case [2]. It is the result of a collision of two galaxies. In such cases, you expect (a) the stars to "fly through" each other, as they are basically point masses and only interact gravitationally, but (b) the gasses and dust to collide. In this case, most of the mass is in fact gas and dust, so it should be distributed in the middle of the cluster (having just had an inelastic collision). But in fact observed mass is where the stars are, further out of what looks like center of mass.

[1] https://www.nature.com/articles/d41586-022-01410-x [2] https://en.wikipedia.org/wiki/Bullet_Cluster

But GR is complex enough to allow very different behavior depending on the initial conditions of galaxy formations. So it would not be surprising to see different scale of deviations from Newtonian mechanics depending on Galaxy history or age.

[1] - https://arxiv.org/abs/1810.04445

You are saying that without another theory to explain something, the current theory must be true because ... How does this logic work exactly?

In the middle ages, there wasn't a theory that explained headaches, so dismissing the idea that we should cut hole in people's skulls to let out the evil spirits is "stupid"?

Dismissing the idea that lighting is caused by Zeus being angry is "stupid" because there is no other theory yet?

We know that General Relativity is incomplete. It cannot describe the work at high energies or very small scales. It is useful as far as it goes but it is not a perfect match.

Quantum Mechanics does describe the world of the small and the hot, but we cannot apply it to the larger world. QD, too, is incomplete. We look for a new theory that can combine the features of both but it evades us, so far.

Dark Matter is a similar theory. It describes a world. It seems to describe this work better than other theories like MOND. Looking for actual dark matter will let us see if DM is a good enough theory or not. If another theory can be found that is a better match, we can switch to that. In the meantime, every time we continue to try to verify/disprove DM.

“Dark matter” is just a phenomenological name to whatever mechanism that might explain all the observations consistently. Under this banner, people have studied models of everything from axions to small black holes. Speaking as someone who’s worked in the field, nobody is “ignoring” alternatives to dark matter — if they could think of ways to make up new alternatives (leveraging MOND, or otherwise) they totally would do it — the problem is that most such alternatives raise more complicated questions like breaking the consistency our theory of gravity, or introducing bizarre quantum behavior of certain cosmological fields, etc. Maybe all those problems can also be solved — but nobody knows how! That’s why it appears like they “ignore” it.

Further, any new theory going to supplant dark matter needs to explain not just galactic rotation curves, but also beat the Lambda CDM model in its ability to model/predict details of the cosmic microwave background radiation. AFAIK, MOND or whatever else is not even close, when you try to put all that together.

To me as a lay-man, this seems like you are indeed ignoring non-dark matter solutions, as dark matter allows you to add various magical elements to explain up whatever is missing in other models. This seems very un-scientific.

It’s not like physicists are looking for every little possibility to be fanciful. Here’s the irony — The “DAMA” experiment has been claiming to have discovered dark matter particles for years, but most physicists don’t trust that the experiment is understood well enough to call it a victory. The process of “adding particles” is an extremely rigorous one (the various detailed checks and stringent requirements involved are unsexy content ignored by pop science). Adding particles is how we discovered the Higgs boson, the weak nuclear force carriers and the internal structure of the proton. It is a battle tested process that underpins the tremendous success of particle physics in the twentieth century.

> To me as a lay-man, this seems like you are indeed ignoring non-dark matter solutions

Seems like there is a massive disconnect with how one actually “does research”. Let’s say I completely agree with you and want to come up with alternative models, and I’ve read the literature — but don’t know how to solve the hairy outstanding problems. What do I actually do when I get up and go to work tomorrow morning? :-P

I don't think anyone is disagreeing with this. However, the current observations are best explained by dark matter. The various MOND models don't fit as well. We either actually detect and confirm dark matter or someone comes up with a MOND model that fits as well or better and has testable predictions.

When people deride MOND, it's not to say that it can't be an answer later but people who are pushing it now are all ignoring some gaping hole in their theory.

Continuing research is fine but, confidence wise, dark matter is in the lead by a lot.

If dark matter distribution weren't arbitrarily assignable, why would reputable physicists suggest that some galaxies might have a dark matter clump at the center and not a supermassive black hole?

The evidence for a highly homogeneous early universe has basically been refuted, unless systematic difficulties in assigning galaxy distances (certainly a possibility) JWST is observing galaxies that are forming WAY too early for a standard gravitational model; MOND has predicted early galaxy formation for more than a decade now.

Something has to explain why galaxies aren't all exactly the same right? Whatever other theory out there will have some "free parameters" that explain that difference won't they? The observable matter differs, why shouldn't dark matter?

I think you're being really cynical about the scientific process here. The process is a new observation comes along that's a bit out of line with the norm and scientists look at the parameters and see if Dark Matter is still a candidate to explain it. Do you think it's unrealistic to assume that different galaxies might have different amounts of dark matter (should it exist)? They have different amounts of observable matter. If there was some galaxy that just didn't make sense by just adding some arbitrary amount of dark matter, it'd put a real damper on the theory but that hasn't happened yet. As of today, the only thing that seems "off" about galaxies is the amount of gravity which is very simply explained by adding in some amount dark matter.

I think my intuition of that is probably the same as your, that it seems kind of ridiculous to just add some invisible matter to the equation but it does actually do a really good job of explaining a few different phenomenon. I suspect the process for the acceptance of dark matter initially began as a "these galaxies are behaving _as if_ they had more matter" and a lot of people trying to explain why that might be to no avail until they kind of just accepted that maybe there just _is_ more matter that we don't see. At least that's how it's been for me. I'm still holding out some kind of hope/expectation that there's a better theory out there that actually explains dark matter/energy in a more satisfactory way.

The analogy would be if we didn't figure out GR, used dark matter to explain mercury's precession, then used dark matter to explain aberrant voyager probe kinetics, then rejected GR because "it failed to explain the weird motion of voyager"

Adding invisible matter is not crazy, but over time you make your model increasingly unfalsifiable as the observations fail to meet the expectations. Ideally that would decrease confidence in the model, but for dark matter it seems to have entrenched the conviction of mainstream astro.

Change that "must" to a "may" and you have something that matches what was written in GP.

The impression a lot of people have about dark matter is I think similar -- dark matter / dark energy basically comes up as an explanation for why theory doesn't match what we observe. Thus a lot of people assume there is simply a more reasonable explanation, like a hidden variable we haven't considered, that would result in a theory that matches our observations. When I was in high school physics this was my assumption as well. I think if that were really the problem, though, someone would have solved it by now. That said, never underestimate an entire school of thought's inability to see things differently.

> (b) We need to scientifically understand why the dark-matter based model, being the most falsified physical theory in the history of humankind, continues to be religiously believed to be true by the vast majority of the modern, highly-educated scientists. This is a problem for the sociological and philosophical sciences and suggests a breakdown of the scientific method [18].

There are few hot takes I agree with more. I don't care if the opponents of DM/DE are proponents of MOND, or proponents of something else involving pixies and turtles. We should continue to investigate DM/DE, but not with the religious fervor that we do today.

It's not true that there are no other theories; here in Norway we believe that it's caused by Thor hammering away up there. Some say he's hammering away at dark matter...

> Dismissing the idea that lighting is caused by Zeus being angry is "stupid" because there is no other theory yet?

I think you could have picked far better examples.

Hopefully I'm not misrepresenting his views, I think the paper is DOI:10.1140/epjp/i2011-11032-x

> any curved Riemannian space must be a subspace of a larger flat host space

This is not even wrong. Yes, you can always embed a Riemannian manifold in a Euclidean space. In fact, there are infinitely many ways to do so. But this is extra structure that you can choose to impose, if you want to - it's not intrinsic.

You might as well say that every string must be a substring of a larger "host" string.

> the 4D Riemann-Christoffel curvature tensor is identically equal to a geometrical tensor associated with the complementary subspace of the host space

Impossibly vague. Yes, sometimes things are equal to other things. What tensor, and associated how?

> Einstein’s field equations are automatically geometrized, with the stress-energy tensor expressed in terms of the contracted complementary tensor

This is a tautology. The Einstein field equations relate the Ricci tensor to the metric and the stress energy tensor, and the Ricci tensor is a contraction of the Riemann curvature tensor. This remains true even if you rename the Riemann curvature tensor "the complementary tensor".

But the deeper problem is this: the entire point of general relativity is that physics does not, in fact, care what coordinates we use. And this is built into the mathematical structure of the theory - there can't be any such thing as the "the coordinate basis vector approach to tensor calculus", because tensors (properly speaking: tensor fields) are, by definition, coordinate-independent objects.

General Relativity largely describes how geometry is nature - what may seem obvious from a mathematical perspective isn’t always so from a physical one. The results of using basis vectors for derivations aren’t riddled with artifacts of any particular coordinate system, as it would become immediately clear when expressing Christoffel symbols, for example - they wouldn’t be equal with other approaches.

One of the other students who was getting her PhD specializing in GR said that using a vector approach gave her intuition about concepts she had previously considered incomprehensible.

But the Christoffel symbols are artifacts of a particular coordinate system - the real object is the connection, of which the Christoffel symbols are just basis-dependent components.

The arguments against it sounded plausible at the time.

There are many more that DM doesn't address, or only addressed by extending it with parameters to fine-tune the result, a process which can also be done with MOND. I hate to keep posting this, but it's a good thorough review of the evidence for/against both DM and MOND, and DM does not fair as well as you imply:

From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm

Couldn't it be that that these dents are balanced out by bulges between the masses, that would additionally "push" smaller masses (like solar systems) towards larger masses (like the centers of galaxies)?

In an experiment it would look like this: the rubber sheet seals the top of a container filled with water. Now if you push down at some point, the increasing water pressure will push up all around that point. If you now added a marble onto that bulge, it would roll down a steeper angle than it would in the normal experiment ... just like if there was extra mass ("dark matter").

https://backreaction.blogspot.com/2021/11/does-anti-gravity-...

> Because I had this idea that anti-gravitating matter could surround normal galaxies and push in on them. Which would create an additional force that looks much like dark matter. Normally the excess force we observe is believed to be caused by more positive mass inside and around the galaxies. But aren’t those situations very similar? More positive mass inside, or negative mass outside pushing in? And if you remember, the important thing about dark energy is that it has negative pressure. Certainly if you have negative energy you can also get negative pressure somehow.

> So using anti-gravitating matter to explain dark matter and dark energy sounds good at first sight. But at second sight neither of those ideas work. The idea that galaxies would be surrounded by anti-gravitating matter doesn’t work because such an arrangement would be dramatically unstable. Remember the anti-gravitating stuff wants to clump just like normal matter. It wouldn’t enclose galaxies of normal matter, it would just form its own galaxies. So getting anti-gravity to explain dark matter doesn’t work even for galaxies, and that’s leaving aside all the other evidence for dark matter.

She goes into a lot more detail in the video (the blog post is just a transcript), but hearing her describe the thought process is interesting because she spent a lot of time on this theory (I’m assuming after she got her phd), so you’re in good company for coming to this initial conclusion at least :)

Just like entangled particles that are lightyears apart, when you measure the angular momentum of one, you know the other's because of conservation laws and not because of some hidden object nudging the particle the right way either.

Spacetime bulging the other way ("anti-gravity") would simply be a feature of spacetime to conserve some other property. In my example that would be the water pressure that wants to stay constant.

Some commenter below mention that this is like a waterbed. I like that analogy. If you put a large marble on a waterbed, not only will it create a trough, it will also ever so slightly lift everything around it.

https://science.slashdot.org/story/20/11/22/1714220/to-expla...

In your analogy rubber sheet analogy, it's about stacking another rubber layer on top of current one.

It has no prediction powers , but it would explain things like why gravity is so weak and why we can't detect it.

ie the observations seem to show more mature structures than predicted, we're finding.

I've been wondering if some MoND / non-DM kind of theory may better explain these new JWST observations.

- I'm not anything close to an astrophysicist, so give me some rope :-)

Specifically this paper[0] is mentioned as predicting it

> Remarkably, the data roughly follow the green line, which is an L* galaxy magically put in place at the inconceivably high redshift of z=10. Galaxies seem to have gotten big impossibly early. This is why you see us astronomers flipping our lids at the JWST results. Can’t happen.

> Except that it can, and was predicted[0] to do so by Bob Sanders a quarter century ago: “Objects of galaxy mass are the first virialized objects to form (by z=10) and larger structure develops rapidly.”

0: https://arxiv.org/abs/astro-ph/9710335

A MoND-like theory that allows the strength of gravity / inertia to vary over time can explain that. Granted, that sounds farfetched, but then, so does DM. Of course, it's easy to spend money looking for DM, and hard to spend money looking for a better theory of gravity.

https://news.ycombinator.com/item?id=29488993

> the observations seem to show more mature structures than predicted

Thank you for closing the loop for me! (would you happen to have a source?)

Probably. Certainly non-MoND people have:

> so give me some rope

Ok, here's some rope <https://telescoper.wordpress.com/2022/08/10/recalibration-of...>

Telescoper is an anagram of Peter Coles. It's his blog. He's a well known physical cosmologist <https://en.wikipedia.org/wiki/Peter_Coles>.

Adams et al., authors of the preprint linked in that blog entry, are all well-known astronomers. Adams is a research associate and Observational Astronomer at <https://en.wikipedia.org/wiki/Jodrell_Bank_Observatory> specializing in high-redshift galaxies, and is in several large-telescope international collaborations (ESO, SKA, MIGHTEE, LADUMA).

The punchline: those seemingly super-far-away galaxies are getting much much closer as JWST's calibrations proceed according to plan.

This paper also adapts GR in such a way that it is consistent with galactic rotation rates, the anisotropies of the CMB, and cosmological expansion -- while showing that the simple operation of gravity is the cause of each of these phenomena.

Cyclic Gravity and Cosmology (CGC) predicts that there are discrete specific sizes allowable for macro-objects. The instability of Bennu and the fact that it behaves more like loosely held scree rather than a compact mass -- is an example of a mass that is not exactly at one of the discrete allowable sizes. Please also refer to the link I included wherein I uploaded a video simulation of the formation of a solar system using this type of force law. (This is in section 18 of the paper)

I would greatly appreciate any comments on this idea. Copies may be downloaded here:

https://vixra.org/pdf/2203.0032v3.pdf

I think many people would be happy (myself included) if there is theory that wouldn't require dark matter and dark energy. But at the moment there is none and to me (and many astrophysicsts) the universe with dark matter is the best description we've found so far of what we observe.

I do think dark matter is tantalizing because it hints strongly that our model needs a fundamental change in understanding at some other level. It just feels like too much wall paper to be adding to the model in an attempt to cover the holes.

Doesn't mean I find any of the current alternatives more viable then our current models.

This isn't to say that is a bad thing, it is a great thing.

But I think what you mean that you treat some theories as axioms (temporarily, I hope). It's a great aid in thinking but a great source of confusion for newbies as well.

Kepler and Newton's models produce better predictions over a long period than Ptolemaic models, which is great. But it's a bit of delusion to think that necessarily means they are analogous to the cosmological mechanisms that produce orbital mechanics.

this was my point with the initial epicycle comparison

It's incredibly hard to know until we've either observed DM particles or made good alternative hypothesises which also fit the data.

Dark matter, on the other hand, is not a conceptual necessity but an empirical one. Whereas aether was something people clung to because they couldn't imagine a world without it, dark matter is something scientists have been forced into even though the world would be more conceptually elegant without it.

The reason we teach the Michaelson-Morley experiment in basic Physics classes is because it's a bedrock example of Science functioning correctly. The reason people believed in the ether theory is because in the wake of Maxwell's revolutionary work, there needed to be an explanation of how light propagated that could be reconciled with Newton's mechanics which had been laid out 200 years earlier. In hindsight, we credit Einstein with finding the way out, but in the late 1800's it was in no way obvious that it was going to work out that way.

I don't think that's case, at least to me it seems that the basic objection is some vague discomfort with the idea, not an issue with the falsifiability. And dark matter is clearly falsifiable, it has already survived many observations.

The CMB power spectrum would look entirely different if there was no dark matter.

Gravitational lensing in certain areas in the sky wouldn't look as strong as it does without dark matter either.

Going back to my Einstein example, there were other Physicists who contributed to Relativity (Lorentz, Minkowski, Poincaré, Fitzgerald) but we give Einstein the most credit because he had the right idea, that it's not enough to treat the speed of light as a constant in your math, you actually have to believe the Universe works that way. One of the stories that gets told is the reason Einstein never got a Nobel prize for Relativity is because the Nobel committee just wasn't ready to give it up and credit Einstein with discovering a new, fundamental truth about the Universe.

Another story that gets told is that when Einstein was an undergrad at the ETH in Zurich, Minkowski was one of his professors and he dismissed Einstein for being a "Lazy Dog" because the math never came easy to him. But he looked at how Physics was taught at the time and knew something had to change. And he was right about that.

I'll just leave this here, Feynman explained it better than I ever will.

https://youtu.be/NM-zWTU7X-k

The aether makes specific predictions. Like the fact that we're all traveling through it because it is a background medium. These were clearly refuted.

Fields are not aethers. Fields do not have a reference frame.

I mean, i imagine its difficult because we can't really conduct direct experiments, but i don't see any reason that its non-falsifiable.

It's popular to look for types of Dark Matter that interact very weakly with forces other than gravity (WIMPs). That's a lot like the joke of the man searching for his keys under a street light even though he dropped them away from the light, because the light is better there. It's perfectly possible that Dark Matter ONLY interacts gravitationally, and so will never be detected by any of the current approaches. Detecting the gravitational force of an elementary particle is ridiculously difficult, well beyond the sensitivity of any current detectors. Even if we could, there would be a huge noise floor from all the normal matter around!

I don't know which aspects you think dark matter shares with miasma. Dark matter fits all the data; miasma doesn't.

the fact that dark matter is widely accepted without proof, and that the current model continues to encounter new anomalies, puts it about on the same level as strings for me—very cool and i hope it's real, but likely to be explained away when we better understand quantum physics and higher dimensions

And dark matter is not accepted without proof. It fits the data extremely well. That is the proof. It explains many unrelated phenomena and isn't ruled out by anything; just like gravity. The fact that we can explain why things fall down, why the moon orbits around the earth, why the earth orbits around the sun, why stars form, why planets form and a whole bunch of other phenomena with one force is the proof; we haven't seen a gravity particle, but that doesn't matter, because that's not how proof works in science.

It's not about thinking it's cool and hoping it's real either. It's simply the only explanation we have for a whole bunch of stuff, and it's a damn good explanation that's extremely simple.

That's not to say it's necessarily correct. It's just the only explanation we have, and we have no reason to believe it not to be correct.

> we can explain why things fall down

that's pretty new too! and it's not semantically correct, but was considered so for quite a while. this is a pretty good analogy for why i'm skeptical of dark matter

Suppose you had six frames of a ball moving in a parabolic trajectory. A sixth order polynomial would fit the data incredibly well, but I don't think you would accept it as a good model for the motion of the ball.

You can't say "it isn't ruled out by anything":

EFE rules out LCDM. Galaxies are redshift >~ 7 rules out LCDM (we now have galaxies at redshift > 13).

DM's failures are more numerous, and it has to be extended with new assumptions to account for other observations. MOND's predictions fair better as favourable evidence actually [1].

[1] From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm

More to the point, since epicycles are just a Fourier decomposition, they're actually perfectly correct. The problem, though, is that they are a nightmare to compute with unless you have a modern computer.

Once Kepler decided to use an ellipse to model solar orbits; however, things were vastly easier to compute. At that point, the epicycle model got swiftly pushed aside.

The reason why LCDM gets called epicycles is that there is charitably one free parameter per galaxy (dm: bm ratio -- but actually more, because it's an arbitrary distribution)... So the fact that it has high explanatory power is unsurprising. And what's should be dismaying is the number of things it can't explain, given just how many free parameters you're allowed to have in a DM model.

Where we run into trouble is by assigning too much credence (or too little skepticism) for that which has not been proven yet. This can be counter-productive as it may stifle research into other areas (in this case gravity). When we take the "best current theory" and deride skeptics of it to the point that it discourages other areas of investigation, that's a bug, not a feature.

Our current understanding of gravity is woefully inadequate, as shown by our inability to reproducibly measure the gravitational constant as well as we should be able to with current technology. The flyby anomaly is another example where DM does not help. I hope that the current popularity of DM does not stifle research into gravity.

I mean do we really believe that we know everything about this? Somehow, we A) know we have large knowledge gaps, AND B) believe that we understand things we have unanswered questions about.

We have a LOT of confidence about dark matter and exactly zero direct evidence.

It seems much more likely to me (I am not a scientist) that we are wrong about things that we assume to be hard facts and those error(s) have artificially produced the need for dark matter in order for things to make sense.

I think this is what the OP is saying though: There are many ideas about what dark matter is or could be, but astrophysicists model the early universe using only baryonic matter, the results _are wrong_.

As a theory, it's quite simple: in its absence, the physics doesn't work. If you add matter that behaves in a particular way, simulations align to observation.

What is dark matter? shrug I don't think we can call any answer to that question a theory yet, we don't have any way to falsify it. We have many hypotheses, but those aren't theories.

That doesn’t make any particular theory correct. It is only proof that our current model is wrong.

Filling the gaps with math, and calling the math “dark matter” does not make dark matter correct; one or more of our existing models is wrong.

They actually don't, numerous extra parameters have to be added for DM to account for observations. DM was invented to fix one problem with observation, and 15 more have cropped up which don't fit.

[1] From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm

I think they expect particles to be thrown out and result with slowdown but dark objects as massive as stars wouldn't be thrown at such a huge rate to observe slowdown, am I right?

I mean, what else could you bring to the discussion by only guessing what is being discussed?

The article just talks about particles and dismisses them.

I see nothing about DM composed of massive objects.

What exactly does your comment bring to the discussion?

Consider the heliocentric/geocentric argument some centuries passed. Contrary to pop-science portrayals this wasn't an issue of Man against Church. The heliocentric view initially had numerous flaws and invalid assumptions. But because scientists of the time genuinely believed in the geocentric universe, they obsessed on these flaws and trying to use those to discredit the view, instead of impartially considering the idea and seeing if it could be patched and whether it would ultimately make more sense.

The "right" answer, with negligible refinement, can often look much worse than the wrong answer which has had decades, if not centuries of mass refinement. And that can drive those with a "belief" to the contrary, to want to attack it. The analogs outside of science are as endless as those within it.

https://www.forbes.com/sites/briankoberlein/2017/02/15/quant...

https://arxiv.org/pdf/1908.01589.pdf

I'm not here to debate with physics cranks; further replies will not be read.

We do not know that dark matter exists

> * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations.

The stories we like to tell require dark matter. Otherwise our stories are wrong

> * There is a broad variety of Dark matter models that are consistent with simulations.

We have a lot of stories using dark matter. We really like it

> * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO.

Our stories are just stories but it is all we have

> * There are a few cases where you can find tensions with the existing DM based paradigm.

Our stories are not internally consistent.

> * A final point. Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background.

We really like stories with dark matter

I love Astrophysics! Pure math with constraints. Do not confuse it with reality

The stories which persist are useful. Calling them theorems or theory helps us rei-fy stories into propositions which can be used to do science. Science is about modelling things, understanding things, and testing things. It is a valid proposition to argue DM is defined by stories which remain untestable, but the proposition there are better alternatives ignores that to construct stories without DM which are better will demand addressing the problems DM has to exist, to define the blank spaces in other, testable stories. You think Absolute zero exists? It's just a story. We haven't got there, we got close, we observe what happens, but this mythical zero point.. What good is it, if we can't get there? I love temperature, the whole thing depends on a fantasy...

Do not confuse your projection of "what is reality" with reality. I love HN, where people make asserts which don't mean what they think they mean!

Nothing in astrophysics is like that, is there?

To me, thats a strong indication the fundamentals of mass, energy as modelled, were projected in advance of radio-telescopes confirming the theory. isn't that what Astrophysics does?

Perhaps there are better examples. Personally, I find the main sequence circular logic reasoning, but its a model for spectral colour, distance, age and size. I'd kind of like it if we flew a mission to hold a Pantone colour wheel up close, and measure the damn thing properly for size and colour but age is very hard. You're back in models of decay of mass/energy outcomes and how we believe half-life in C14 analysis.. except done as spectral lines at a distance based on one solar mass we have close by to model things on. Plus, we don't know how to make pantone colour stable across 20,000 years of flight time to measure a star up close and by then Humans will have 29 eyeballs seeing into the infrared like mantis shrimps.

Gravitational lensing is pretty cool. I think it was initially theoretical, and it was a long time being proved.

Gravity waves defy detectors. If we can get detectors working that will be immensely cool. I may have stepped off the scienting engine by wanting an outcome, which arguably is not aligned to null hypothesis testing. Maybe we can't detect gravity waves and they remain theory?

Look, its rude. I apologize in advance. I do kinda think you're trolling. Are you trolling? Maybe Dang will be upset at my asking. It's ok to ignore.

Different people have different priors. For some dark matter is more unlikely.

We really like to tell stories where 2+2=4, etc.

Which is why we really like telling stories about it.

My point is, we tell stories about literally everything, so i don't know what point you are trying to make.

If your criticism applies equally to basic facts, it is a bad criticism.

> Biologists understand this. It is very easy to get a biologist to say "nobody knows"

Depends on what you ask them about.

Given we can't observe reality directly, since we ourselves see it though a model, We can adapt the above list to everything.

The question isn't "is it a story" the question is "is it useful for prediction"

Much of astrophysics has been.

> Without such theory, claiming DM doesn't exist is simply stupid IMO.

Expressing flaws in an idea does not require a better one in any way. The search continues.

Expecting others to give up their theories that work well for 99% of the problem, or even only work well for the 10% they care about, just because there's a problem with it DOES require something better AND a reason to switch.

For example, architects (technically stability engineers. Architects don't really design buildings except in the artistic sense) are still simulating Newtonian dynamics on plates that fly through empty space held up by nothing. You want them to change? No problem, but you'll have to make something better and point out why they'd want to use something better ... but, truthfully, it works pretty well.

It also reduces the religious aspect, demoting DM to simply a tool used in specific situations, "we don't have anything better at the moment."

We have radio telescopes that can see all the way down to a few tens of MHz. This is far colder than the CMB, and certainly much too cold for the intergalactic medium.

> Plasma Physics hold ions permeating the Universe as a premise & has physical experiments reproducable in the lab to back up their claims

Yes, plasmas exist. There are lots of them in space. This is completely uncontroversial.

> And can use classical EM equations explain how the stars & galaxies work

Absolutely not. Galaxies are firmly in the nonclassical regime.

PV = nRT

What if the P is very low & V is very big...like it is in space? What would the plasma in dark discharge radiate?

> Absolutely not. Galaxies are firmly in the nonclassical regime.

According to some models. Do these models have reproducible experimental evidence? If not, I don't see how you can credibly be so sure of yourself...

Sure, exotic & speculative math is way more fun than reconciling with physical reality. Without reproducible experimental evidence, the model is non-falsifiable (& non-provable).

In contrast, the Plasma model does have physically reproducible experiments that are able to create galactic phenomena in the lab. Live physical experimentation...not computer simulations using math that has never been verified by physical experiment.

For an ideal gas. Plasmas are extremely far from being ideal gases.

> According to some models. Do these models have reproducible experimental evidence?

Yes, general relativity is extremely well supported by the evidence.

PBS Spacetime covers this topic in several videos, a good one being [1]. Matt (the host, an astrophysicist), describes what MOND (and other alternative theories, like relativistic-MOND) needs to do but haven't done yet:

1. Give the right answer in more than one special case. MOND may describe a few situations accurately, but it does not describe all situations where we have evidence for dark matter. Only some.

2. Consistent with other known laws and theories that are experimentally verified. According to Matt and others, MOND is not consistent with existing laws such as conservation of energy or angular momentum.

3. Make testable predictions beyond what it was tuned for.

Am I missing out on some extraordinary development in MOND and/or MOND-like theories that occurred recently?

[1] https://www.youtube.com/watch?v=0sTBZ2G4vow

> The current cosmological model only works by postulating the existence of dark matter – a substance that has never been detected, but that is supposed to constitute approximately 25% of all the universe. But a simple test suggests that dark matter does not in fact exist. If it did, we would expect lighter galaxies orbiting heavier ones to be slowed down by dark matter particles, but we detect no such slow-down. A host of other observational tests support the conclusion: dark matter is not there.

If you're going to claim that dark matter doesn't exist, you need to have a way to explain all of the cases that are known to show that dark matter does exist. Cases like the Bullet Cluster, where a collision left all of the visisble matter in one region, and all of the gravitational matter in another. How can this possibly happen if there is no such thing as invisible gravitational matter? How can there be galaxies like AGC 114905 that don't have any dark matter, if there is no such thing as dark matter for galaxies to have or not?

DM falsehood does not depend on any other known thing being correct. It particularly does not depend on MOND being correct.

The honest fallback position is not "this other thing is better", it is, "We do not know. We had hoped we did, but were wrong." Then, start entertaining hypotheses that encompass all observations, without prejudice, not just favored ones.

Oh wait, that's the definition of dark matter.

The way I see things, the much more prevalent attitude is, "we have a good idea of what's going on, we just haven't been able to confirm our confident assessments with objective data". Very few, if any, prominent people in the field admit, "we don't have any idea what's going on, it is all baseless conjecture that might pan out if 75% of the universe is made of some invisible stuff that we haven't been able to detect in 4 decades despite our best efforts". I'd like to see a paradigm shift from, "we have a good understanding of how things work, now we just need to detect the invisible particles that make all of our formulas work" to "we have really have no idea what is happening, but here is some wild speculation that is the best we can come up with".

A lot of the "dark matter" debate reminds me of the Alzheimers/amyloid situation. Groupthink develops among the establishment in the field, based on very little (if any) objective proof, which ends up discouraging and marginalizing those who pursue other theories.

No, actually calling it missing mass is dark matter. MOND would be another explanation, and there's no reason to conclude it's unworkable.

https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Re...

From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm

Which is why I'm asking about MOND, which I just explained in my comment is falsified from the start (violation of existing established physics).

Why instead of? Dark matter is, so far, the best model we have. Seems sensible to continue research in that direction.

>futile dark matter detection projects.

Why are they futile?

In general, that statement would require defense.

DM also fails numerous observations, like the one described in this article, external field effects, and more. It failed numerous previous observations as well until more parameters were added to DM theories to account for actual observations.

> 3. Make testable predictions beyond what it was tuned for.

This is backwards. DM has been tuned to match certain predictions, where MOND's predictions are free of parameters and yet fit observations. That's actually strong evidence in favour of MOND.

See my other comments here for a reference to a paper that reviews the evidence for/against both.

From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm

You know what else smells like phlogiston? Oxygen. There really is an invisible, odorless, tasteless gas responsible for combustion, plant growth, and breathing. 17th century chemists thought that it was released during combustion, absorbed by plants, and expelled by the lungs. Turns out it's absorbed during combustion, released by plants, and taken in by the lungs.

Phlogiston was a reasonable theory, well supported by the available evidence at the time. It was also wrong. Most theories are. But this could not have possibly been predicted a priori.

That of course doesn't say one way or another that such hypothetical particles can account for the amount of mass needed.

The exact problem is that sprinkling extra mystery mass everywhere produces results inconsistent with observation.

No theory fits perfectly, but that is what we expect in a world of imperfect knowledge.

So far modified gravity theories can’t explain everything. That and they would have to integrate with general relativity for which evidence is only getting stronger.

Is that hypothesis correct? (i.e. would we expect lighter galaxies orbiting heavier ones to slow down if dark matter exists)

If it is correct, and we don't observe that effect, would it not be a very strong argument refuting the existence of dark matter?

For folks mentioning this as a "crackpot" or "anti-dm" theory, is the strong claim that the hypothesis is incorrect?

Would someone who relied on Newtonian dynamics 100 years ago likewise be incorrect to do so because it failed to explain many aspects of planetary motion?

It is well known that the relevant wake friction calculation ("classical Chandrasekhar") is for a pointlike structureless body. Satellite galaxies and their parents are not this. Importantly satellites can lose matter and thus mass, so one has to correct for the evaporation and tidal loss (especially to the halo) of the satellite galaxy; and the host galaxy's potential grows adiabatically over the lifetime of the system.

If the density of the halo and other sparse matter increases closer to the host galaxy, inward migration of the satellite sees two effects driven by the collision into more particles: more satellite mass is torn off by tidal interactions and more halo particles surrounding the host are swept into the satellite's wake. All that matter stays well above the surface of the host galaxy rather than descending into some inner halo or deep into the host galaxy itself. This serves to "suspend" the sinking satellite, posing a challenge for the "they should sink" reasoning; it also reduces the amplitude of the wake, which moderates the drag on the satellite. The combined result is that the evolution of specific angular momentum may not be enough to clearly support Kroupa's claim in the fine article linked at the top.

There has been a lively dialogue in academic publications about the scale of these effects for some twenty-five years. Kroupa's article touches upon his own position in these, but not those of others (e.g. HS Zhao, Binney) who work and publish in the area (with all of the above citing one another from time to time).

I don't mind this at all, personally, as at worst Kroupa is omitting something an interested reader could hunt down her- or himself, and after all it was "only" published on a general interest site whose present front page (https://iai.tv/articles) has an article by Eric "electric universe" Lerner entitled "The Big Bang Didn't Happen".

The view you provided helps understand the counter argument of why competing factors (e.g. losing mass) result in more sophisticated outcomes than "we would expect lighter galaxies orbiting heavier ones to be slowed down by dark matter particle". I still don't know enough about the subject to understand if the original authors claim and expectation is still likely. But your context helps me understand other factors that are in play.

Especially DM proponents!

What I do have is a metric fuckton of experience with incentives and mechanism design in technical fields: and mainstream physicists have been talking like people who have bonuses linked to specific outcomes for decades.

The moving goalposts around falsifiability, the too-coincidental synergies between supersymmetric-style (and there for string world) stuff and missing mass (dark matter), the growing group of highly-qualified skeptics, the curt public dismissals everything from LQG to constructor stuff to MOND as beneath refutation, the popularizers who go on PBS and don’t even acknowledge a debate.

And the alternative explanation, that it takes so long to learn this stuff that your career is over if Dark Matter or String Theory turns out to be hokum? Yeah, that scans.

There are simple situations where physics can be reasonably distinguished from the noise. Think Newton watching an apple fall from a tree -- these are test harnesses with a debugger and careful control of the environment. There are also astrophysical situations where it's very hard to squeeze insight out of the chaos. Instead of looking at an apple to derive Newtonian gravity, imagine instead trying to figure gravity out by looking at a flock of birds. There are tricky things in the way of our understanding, like lift, turbulence, and biomechanics. We hope to understand the bad situations someday, but right now it's tough. Instead of the nice test harness, these situations are like debugging through cryptic, un-reproducible user complaints. What's their OS? Do they have the right drivers installed? Do they have all of their ports blocked for some reason?

In astrophysics, one nice system is the Universe at very large scales: echos of the big bang, the clustering of matter, and the formation of structure. We have great data these days about the large scales, and general relativity (GR) + dark matter (DM) is a very predictive model here. To our chagrin, the data always matches the theory. It takes 5 or 6 parameters, but the model has withstood huge improvements in data quality without really changing since the 90s. The other nice situation is the small scales like our own solar system: we can measure things extremely precisely at home, and again GR works remarkably well. There are a few other good situations involving objects like pulsars. Finally there are the hard situations: intermediate scales that involve the messy physics of star and galaxy formation. We don't really know how these processes work, but we can cobble together simple models with DM that sort of match the data.

Modified gravities in the literature always seem to act exactly like general relativity + dark matter in the clean, understandable situations, although it takes various contortions (screening) for these models to do this. Modified gravity models seem to only act differently in the bad arenas that are hard to understand, when there trickier issues like galaxy and star formation. That's deeply suspicious, and really weakens the value proposition of these alternatives. It's like introducing a software feature to fix something that never happens on the dev machine, based on mysterious user reports that you can only half decipher.

Five or six freely chosen parameters is a lot. You know the von Neumann quote. If those parameters have continued to fit vastly more precise measurements without changing for 30 years that’s starting to sound like failure to falsify.

And if so, ultimately you folks have posed the particle/high-energy folks a problem: we found a shitload of weakly/non-interacting mass, where’s my particle bruh?

To the layperson, exotic undetectable mass sounds like something that will turn out to be a good metaphor for something deeper, but that doesn’t mean a lot via the definition of layperson.

Even laypeople know the string folks are full of shit: Witten has a Fields and not a Nobel for a reason.