I think it's like this: Imagine you have a cloud of dark matter, with a combined centre of gravity. Now imagine a "piece" of dark matter falling down to that c

I think it's like this: Imagine you have a cloud of dark matter, with a combined centre of gravity. Now imagine a "piece" of dark matter falling down to that centre of gravity. It doesn't impact, or slow down with friction, or anything like that, it just keeps going, through the centre, and out the other side, just as far as it fell before. There's no way to clump together, or stay in the centre.