SOMETHING affects the rotation rates of stars in galaxies. That's the root of the proposal for dark matter. This is called the "flat rotation curve".

However, the one provable fact that seems to illuminate how bad the working theories are is that every galaxy has the same anomalous star rotation rates. All the curves are flat.

Doesn't matter how old, how big, how distant, etc, the galaxy is. Every single one of them have rotation curve at a rate inconsistent with its observable mass; but all of them have flat curves.

None have curves rapidly falling off the further a star is from the core. None have curves rapidly increasing the further a star is from the core.

In other words, to make the dark matter theory work, each galaxy in the universe has to be surrounded by a halo of exactly the right amount make the rotation curve flat. None have more, none have less than the amount required for a flat curve; and the amount required is different for every galaxy.