These are very normal, nearby elliptical galaxies that they studied, and if those galaxies don't have dark matter it calls into question the whole theory of cold dark matter.

Galaxies were mere tide pools. The empty space between them was a killing field of dark-matter predators and electromagnetic anglerfish.

We know less about the structure of our own galaxy than we do about many nearby galaxies like Andromeda. We literally can't see the forest for the trees because we are embedded inside our own galaxy, and interstellar dust blocks our view.

If we see a star at a large distance from the center of the galaxy, that star is going to be mostly moving either away from the center or back toward the center. Almost certainly, most of its motion is perpendicular to our line of sight.

In the merger process that produces these galaxies, a lot of the stars get flung out to fairly large distances, and they end up in highly elongated orbits that take them far away and then back in close to the center.

You cannot measure the absolute speeds of the stars, but you can measure their relative speeds along the line of sight, because if a star is moving toward us its light is shifted to shorter wave lengths, and if it is moving away from us its light is shifted to longer wave lengths.