Galaxies rotate as far as we know as quasi-rigid objects.
Physics tells us that the stars closest to the center should rotate faster, those further away slower. The point is that they do not manage to do so, many close to the center rotate more slowly than they should and many far away much faster than they are supposed to.
The solution or patch has been to come up with something called “dark matter” that nobody knows what it is and it is supposed to add the missing mass for that to happen, and it is supposedly invisible. Come on, it is transparent to all electromagnetic radiation.
Either that, or gravity on large scales is not what we think.
And at the moment we are between one of those two things, but without having proof of it
