It has been observered that spiral arm formation galaxies exist. This is a problem because due to the laws of gravitation, stars close the galactic center should orbit with a much higher angular velocity than stars at the distant edge of a galaxy, and so any structure, spiral arms included, should smear out after one or more revolutions of the stars and become a disk galaxy.
In order to explain this unexpected observation, cosmologists hypothesise that something is causing the stars to all orbit at constant angular velocity, so as to keep the structure, spiral arms, intact. The name given to the "something" that causes this is Dark Matter.
There are a number of problems with this hypothesis. Specifically, when two galaxies are near each other, the "matter" that is maintaining constant angular velocity in one galaxy should also be having an effect on the neighbouring galaxy, yet we see no such effect.
Similarly, if the "dark matter" is really a field effect, then the field that causes constant angular velocity for stars in one galaxy should also be having an effect on the neighbouring galaxy, yet again, we see no such effect.
So, given this, is it sensible to consider the term "dark matter" to actually mean matter, or even a field effect. Or is it sensible to consider "dark matter" to be some as yet unidentified effect?