I don’t really understand. Dark matter is in most galaxies interacting gravitationally, but not these three. Their idea is that somehow there was a collision event which stripped the galaxies of their dark matter, because the regular matter got tangled up but dark matter somehow didn’t.

But if dark matter is also interacting gravitationally, why wouldn’t it also be tangled up? And is that something we have seen in other collisions ever?

the Bullet Cluster makes for a good comparison, there's a good image of it in the article. the bulk of the 2 galaxy clusters continued on but left behind 2 clouds of inter-cluster gas

my first impression was this could actually be a counterexample, because there we can see the heavy, dark matter-full clusters continuing on their trajectories at full speed whereas the gas slowed down and is being left behind. compared to these 3 lightweight galaxies which seem to be alone, rather than trailing behind their dark matter-hosting parents

but perhaps these 3 _are_ going slower than their full-scale origin galaxies. I would stake my view as a layman on whether anyone can find the dark matter galaxies that spawned these farther along their expected trajectories

> But if dark matter is also interacting gravitationally, why wouldn’t it also be tangled up?

Friction between regular matter causes it to lose energy and thus velocity. Dark matter doesn't have self-interaction outside of gravity, so it just keeps going.