The science series currently on TV in the US called WORMSHOLES, narrated by Morgan Freeman (the actor) has a number of recent some new ideas. (Very worthwhile series, by the way.) The show last night discussed dark matter and dark energy and referred to a distant pair of galaxies colliding. The show pictured (must have been an illustrated computer simulation ) the new rotation of the combined stars around a new center of mass. But the fascinating thing was the representation of dark matter: the dark matter from each galaxy passed right through all the matter and dark matter of the other since, it does not interact either with normal matter nor itself. So the dark matter kept largely on it's original course despite the collision of the normal matter. They referred to the collision as a "bullet" collision because of the snub nosed shape of the dark matter emerging after the collision....They did not discuss the gravitational effects of the dark matter in this collision yet that must have some effect on all the matter. Has anyone come across such an illustration or proper discussion of such a collision? In other words, the new rotation of the combined galaxies must be affected by the new location of the dark matter as well as the combined gravitational affects of normal matter.