Cosmologists propose there must be dark matter present to somehow change a galaxy's rotation pattern from what is expected.
When there is no visible matter around to be the cause the matter must be dark.
I found a news story from a few years ago (attached below, with a link to the paper) about an analysis of these rotation curves in M31.
from the story:
While conventional models with Dark Matter are able to reproduce the rotational velocities of the inner portions of the galaxy, they have not explained this outer feature and instead predict that it should slowly fall off.
When a magnetic field with this value is added into the modeling equations, the team found that it greatly improved the fit of models to the observed rotation curve, matching the increase in rotational velocity.
This description must lead to this conclusion:
When cosmologists consider magnetic fields in their model the results are better than when those fields are ignored.
I can only wonder if this is the fundamental problem with dark matter.
Magnetic fields are not visible matter but they affect charged matter, so the fields are dark.
If these fields are ignored then the actual cause of an unexpected change in motion is probably missed, leading to the theoretical dark matter as the invisible cause.
Before dark matter is assumed to be the final solution invisible electromagnetic forces must be considered first. Otherwise the initial conclusion might be wrong.