A lack of friction.

 

It is friction that damps orbits enough for massive compact objects to form. Since dark matter presumably only has a weak interaction there is no effective frictional force.

 

So, dark matter has gravity, but no friction? Doesn't that mean that it would be impossible for matter to clump together to form macroscopic objects? With the attractive force, but no impeding force, it would seem that a system of dark matter particles could be in somewhat of a perpetual harmonic motion. Is that reasonable?

Aren't ions in crystals of normal matter continuously vibrating anyway?

An interesting idea...

 

MrMongoose, this would be completely different to that as it'd be on large distance scales... and they probably don't interact with EM, hence the no friction...

But they could radiate gravitational waves, so there would be friction. Just very weak.

Odd that it isn't all just clumped together in one place.