We think we understand gravity pretty well. So astronomers think they can look at a galaxy, make measurements of its size, shape, and rate of spin, and then calculate how much mass it has. The problem is, that calculated mass is generally about 20 times the mass of the stars we can actually see. The rest is called "dark matter," which is a cute way of saying "we have no idea." Some of the dark matter may be black holes, but the calculations show that most of it has to be spread out over the entire galaxy in a very even way. So dark matter remains a mystery.
A new set of calculations about the effects of dark matter, which I won't attempt to explain, makes the behavior of dark matter even weirder. It seems to act in some sort of exact correspondence with the matter we can see. But if dark matter is some sort of stuff, why is that stuff distributed in exactly the same way as the visible matter? It makes no sense.
I would say that these new results support the notion that dark matter is not a thing, but some sort of force generated by regular matter, or, even more likely, a sign that we really don't understand gravity as well as we think.