Dark matter sounds suspiciously like aether to non-experts. I’m not saying it is, but do recall that the concept of aether filling the universe was an immensely popular one prior to the turn of the last century, so not too long ago. We don’t believe it any more, it’s hogwash of course.
Aether seemed to be necessary conceptually because the theory of electromagnetism was a theory of waves, and no would could conceive of waves except in some medium, hence aether. Einstein's breakthrough was to show that the concept of aether was not necessary to explain empirical observations about electromagnetism and the speed of light.
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 luminiferous ether theory met the basic criteria of being testable and falsifiable, hence the Michaelson-Morley experiment. Isn't that the basic objection to dark matter, that there's nothing in it that's falsifiable?
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.
> Isn't that the basic objection to dark matter, that there's nothing in it that's falsifiable?
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.
Until Michaelson-Morley’s landmark experiment, we hadn’t found a way to falsify aether either. Is there physical proof that dark matter is not falsifiable? If not, we still have a bit further to go.
There is always a tension in Physics between physical proof and observability on the one hand, and having the right idea, the right intuition on the other.
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.
Dark Matter in totality is hard to disprove (as it's really not one theory, but a rather large set). But large parts of that parameter space is testable, and is tested. See WIMPs for example, where we searched extensively. Have we ruled out everything? No. But we have ruled out many variants, mass and coupling ranges.
Michaelson-Morley didn't falsify aether. At the time, their results were interpreted within the aether framework in terms of compression of the aether yielding the Lorentz transformations.
We absolutely do not have an aether! Light does not travel through a medium like ripples in water.
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.
Lots, and more than we can afford, assuming particle colliders could even produce it.
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 thought that a purely gravitationally interacting dark matter would not produce the right clumping, i.e. there must be an additional force -- either one of the known ones, or a new one.
True, a pure GIMP alone can't do it. But they could account for most of the observed effects of Dark Matter, meaning we'd be very far from ever detecting any. And an extra force (if one exists) might interact with Dark Matter but not to any detectable degree (or at all) with normal matter, leaving things in about the same position (you'd never produce Dark Matter from colliding normal matter).
