Well, the angular velocity of the outer rim of the Milky Way is very much the same as the inner part of the galaxy. The velocty curve, that is velocity of orbiting stars versus radial distance from the core, is flat with little variation. That is not what Newton would have predicted. He would have said, just as the inner planets orbit faster, so should the inner stars of the galaxy. But the outer stars are orbiting lock-step with the inner stars. Herein lies the conundrum.

Dark matter simply states that there is alot more matter (and mass) further out beyond the visible rim, and that we are arbitrarily truncating the galaxy at a certain distance from the core, for example at a distance where no further stars are visible. I think it's reasonable that there is alot of matter beyond that hasn't formed into stars, and therefore isn't visible to short-exposure cameras. It's also reasonable that there could be immense clouds of dust out there too diffuse to block enough light to be visible.

I think they have Newtonian dynamics right. Measuring devices exist to precisely verify its predictions.

But if you want an exotic explanation.... I'm working on it. It goes something like this:

A galaxy arises from a substrate, like a plate glass window. One plate of glass for each galaxy. On small scales the substrate can accommodate some movement, but since a given particle is emergent from one small part, it can't travel far. Therefore the whole galaxy is constructed as if the stars are fixed points on a solid surface, and the solid surface rotates. The many plates of glass each represent a single galaxy and are moving and rotating relative to each other.

Like watching a window shatter in slow motion. Is that artistic OR WHAT !!!!

Here's an interesting consolidation of GUTs based on substrates: Superstring Theory rely on extra unseen dimensions to explain some phenomena. The Holomovement relies on an interconnectedness, and an emergence of reality as we know it from a substrate, like a hologram emerges from a specially-treated optical material. Even if our universe is finite, there can still be a substrate beyond it.

What if we consolidate these ideas? Consider the possibility that the substrate and the unseen dimensions are referring to the same thing. We are like 3-dimensional entities occupying a subset of a 10-dimensional universe. Or. We are like 3-dimensional projections from a hyperdimensional substrate. Same idea. Different words.

I love this stuff.