Yes, to explain it in a bit more detail, the problem is this:
We have, with general relativity, pretty good equations for how gravity works. Scientists assume that gravity is the only force that needs to be accounted for in the motion of the galaxies, since the nuclear forces have negligible effect at stellar distances and electromagnetic forces shouldn’t have much effect either (if space is mostly a vacuum).
So, applying the gravitational equations to predict how a galaxy should rotate should be simple enough, at least to make a rough model. However, when we do this, the models predict that the arms of the galaxy should rotate slower than the center of the galaxy (since gravitational forces weaken as the distance between masses increases). When we look at real galaxies, though, we see the arms rotate at the same speed as the center of the galaxy.
To account for this discrepancy, they invented the concept of “dark matter”. Basically, if they add enough mass in the right places to their models, they can force the model to match the observations. However, this is a completely arbitrary forcing of the equations. It’s too ludicrous to believe that in every galaxy there really is invisible matter allocated in just the right places to make the equations work. The probabilities against such a thing would be astronomical.
On the other hand, if we consider the idea that the motion of galaxies is not determined only by gravity, then there are much cleaner solutions available. Scientists just don’t seem to be willing to abandon the gravity-only models yet, so they continue to putz around with dark matter.
I prefer epicycles.
What else?
Post 45. . .article discusses thoughts on galaxies and planets wandering off.
Not a troll here - looking to understand this better.
So, lets assume we have the galactic center, a star 1 a distance of x from the center and star 2 x+y from the center and in a galactic arm. Lets also assume we take a ray that forms a right angle from a tangent plane from the galactic center that transects S1 and S2, like a spoke in a wagon wheel. As S1 and S2 are observed to rotate around the galactic center, do they stay "within the spoke"? And relativistic models predict that S2 would lag behind the tangent ray that transects S1?
I saw what you did there.