[longshadow]

[Raising my hand ...] I have a question. As these clusters pass through each other, and the visible matter gets slowed down so that the dark matter sails through faster, the dark matter leaves the visible galaxies behind. Fine. Now then, I thought the first clue to the existence of dark matter was that galaxies were rotating as if there were lots more mass than we could detect. So, won't the rotation of these "naked" galaxies (stripped of their dark matter) cause them to fly apart? Is that going to be observed?

I'd need to have a better understanding of exactly what's going on to answer that; I'll have to punt until the heavy hitters show up.

[PatrickHenry]

This opens the possibility that you can have a galaxy-load of dark matter flying around without its visible component. And that's a lot of mass. If our galaxy -- a rather congenial place so far -- were to encounter something like that ... I donno.