I think the puzzle is, the galaxies and the dark matter are only affected by gravity, so one might expect them to follow the same dynamics. However, there is a concept of "kinematic temperature", which is a fancy way of saying 'velocity' of the dark matter particles, and the galaxies. As you all know, when only gravity matters, the only thing that can separate behavior is velocity, so maybe the galaxies had a very different average speed than did the dark matter particles, prior to the train wreck. That might explain why they 'splashed out' farther, if that is indeed what happened. But it's hard to speculate without knowing more of the numbers, like what those speeds are, and what actual simulations would show. I give it 3 months before some computer jock publishes simulations that get the galaxies and dark matter to behave that way, given a clever setup.