Hmmmm... Light matter/energy vs. dark matter/energy...
[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?