xkcd

Looking at it from a quantum field perspective, pretty much. If the only interactions are through gravity then the underlying field's evolution can't be influenced by anything else, I have no real idea what the implications of that would be because we don't have a QFT for gravity.

Well, we know that our understanding of physics isn't correct - galaxies rotate faster than we think they ought to based on the amount of matter that we think is in them based on our theories of gravity and the evolution of the universe.

The "simplest" explanation is that there's a particle that only interacts gravitationally, and has no other interaction with matter, hence being dark. Gravity might work differently on galactic scales, although it's hard to make that maths work; or neutrinos (which are also 'dark') don't have the gravitational interaction that we expect from theory.

Simple answer is that we don't know, and "dark matter" is the useful placeholder term until we work it out. Could be a lot of things, although there's a lot of things that we know it isn't.

Wikipedia has a big list of all the things that don't fit our current model, and which a proper theory of everything would have to explain. Dark matter ticks all the boxes, whereas other theories work for one or two but can't explain the rest.

https://en.m.wikipedia.org/wiki/Dark_matter

https://www.caltech.edu/about/news/dark-matter-flies-ahead-of-normal-matter-in-mega-galaxy-cluster-collision

Here's the best evidence of difficult to detect matter likely being real. You wouldn't see such a shift in gravity if there aren't matter unaffected by friction