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.

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.