[InclinedPlane]

The dark matter halos don't experience friction, and are incapable of emitting heat radiation to cool off and condense.

If you take a bunch of marbles and put them in a big bowl they will roll around for a while but eventually end up in the bottom of the bowl because they keep running into each other. But if you put in special marbles that just pass through each other and don't experience friction then you'll end up with marbles rolling around the bowl everywhere for ever, which is the way dark matter works.