 What this is, we do not know yet, but it could be, assuming this is a real effect, it could be some new kind of force in the universe that has shaped it through its history. It could be that this other stuff that we know is out there called dark matter. Perhaps dark matter is much more complicated than we imagined, that it's able to interact with itself, do all kinds of interesting things. It could even be that our theory of gravity is wrong.