Dark matter is only “missing” in the sense that an explanation of it is missing from established physical theories. It (whatever it is) certainly appears to be present, because it has a major effect on observed galaxy behavior. Otherwise there would be no need to bring up the topic!
I get that, it's not that I don't get the reasoning. It's just that it seems backwards to me (like in the phone example). But maybe my comment was an artifact of the fact that I (layman) haven't really bought into the whole idea that we really ever had any "theory" for dark matter in the first place. To me the appropriate headline would've been more like "astronomers find the first galaxy that actually behaves as theories predict".
I don’t think we do have a working theory that explains all the observed evidence. We know the theory without dark matter is wrong, because the vast majority of galaxies don’t agree with it. But we don’t have a new theory that holds together that does explain the observations, with or without dark matter.
Basically “dark matter” just means “something that makes the observations work” and the choice of words “dark matter” is because it would work to have some kind of yet-unexplained matter that doesn’t interact with the matter we know about. It looked like it might also work to change the theory of gravity, in which case that was a bad choice of placeholder name — but now this is a counterexample to that idea.
I don’t know the math, but my intuition based guess is it’s related to the range of interaction possible with other particles through QMs probability functions. Ie it’s important to remember that at quantum scale matter/energy transfer through space as probability density waves. Dark matter are at a low bar of the interaction distribution. Something about the empirical values of those equations gives way to many more particles at that low end than expected, but their existence couldn’t ever be ruled out.
