The point, as I understand it, is that we've found cases of galaxies that look like they should be about the same mass as each other based on the light they're putting out themselves, but gravitational lensing measurements indicate a significant mass discrepancy. That's what's hard to explain without "maybe there's a bunch of matter we can't see" - if it was more of a "maybe gravity just works differently at galactic scales" situation, that scenario would be impossible.
Ma8ee|6 years ago
I don't see why the scenario "gravity works differently at large scales" couldn't explain the observations until we have some idea how gravity would work differently.
Don't forget that each of these observations rely on a quite large sets of assumptions about everything from how much luminosity from galaxies with a certain (normal) mass can vary to how we estimate distances to very far away objects. We don't have that many observations of e.g. gravitational lensing. And, if we realize we actually don't know how to calculate things correctly with GR, or our theory of GR is wrong, that will have implications for all of those assumptions.
The dark matter hypothesis might very well be right. It is a very reasonable guess. But so far I think the proofs for it have been overstated.
ymolodtsov|6 years ago
And I can say myself that not everyone agrees with that discovery: http://www.astronomy.com/news/2019/09/astronomers-cant-agree...
Well, people still debate about the Bullet cluster. But if it's correct then it's actually a good (relatively) confirmation for some sort of dark matter. It two similar galaxies look the same but have dramatically different mass distribution then there's some mass we don't see yet.