It's useful to add to this that "interacting with itself" in a way that allows clumping implies radiative dissipation of momentum or elastic collision or both.

It's useful to add to this that "interacting with itself" in a way that allows clumping implies radiative dissipation of momentum or elastic collision or both. Without that it takes much longer to ditch enough momentum to fall into an orbit closer to the centre of mass around which it's moving. That is, the non-clumpiness implies that if there is dark chemistry, the reactions must take a long time compared to the present age of the universe.