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Comment by FreeFull | Hacker News Reader

The thing about dark matter is that because it doesn't seem to interact in any way other than gravitationally, there is no way for it to lose energy and clump together. So, collections of dark matter tend to stay rather diffuse. The way you could notice such a collection without any visible matter is through gravitational lensing.

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Why is loosing energy required for particles to clump together?

If they don't lose energy by interacting through some force other than gravity when they get close together, then they will whip right past each other with nothing to slow them down.

Think about two dark matter clouds crashing into each other.

Each cloud just continues with the same speed in the direction it had. They dont interact other than gtavitationally.

After passing each other they start to pull on each other gravitationally and slow each other down. Eventually stop and accelerate towards each other again.

And so they can oscillate forever back and forth, if left to themselves without other clumps. But at no point would they "clump"... they are only close to one another in space when their speeds are most opposite one another. Being clumped means being im the same location and having the same speed.

I expect they would lose energy and clump together due GR effects- the system will emit gravitational waves.

https://en.wikipedia.org/wiki/Gravitational_wave#Binaries

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