I kind of assumed that a YouTube video on that subject would be a waste of time, and boy was it. That was amazingly ignorant. Galaxies are often isolated. They

I kind of assumed that a YouTube video on that subject would be a waste of time, and boy was it. That was amazingly ignorant. Galaxies are often isolated. They come in clusters, and also in groups, and in isolation (including isolated galaxies inside cosmic voids). The isolated galaxies have the same "dark matter" phenomena as galaxies in clusters (which, by the way, do not have perfectly circular orbits, just to start with).