It surely wouldn't be a galaxy then? I think a galaxy has to have stars in order to be a galaxy.
If dark matter is theorized to explain more gravity than there would be without it, that implies dark matter has a gravitational effect, which means it too might coalesce into very dense things. Perhaps dark matter also forms star-like gravitational bodies, but whatever makes it "dark" hides those too.
It seems weird because I'm almost suggesting... Cloaked stars. Which on the face seems terribly unscientific, given what we know about EM radiation of things. But i think dark matter implies things that don't radiate in the EM spectrum.
Part of me wants to speculate that dark matter is a gravitational force echoing from 4th dimensional matter. Like if a cube affected a plane over which it hovered. But that's just me being an ignorant speculator.
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.