When two galaxy clusters collide, the ordinary matter can get slowed down by non-gravitational interactions, leaving the dark matter to continue on inertia and gravity alone.

Closest references I could find: http://phys.org/news/2013-02-dark-galaxies-ways-collision-he...

"In the Bullet Cluster, a collision between two galaxy clusters appears to have caused a separation of dark matter and baryonic matter." - https://en.wikipedia.org/wiki/Dark_Matter

Here are the first links that came up when Googling it:

http://www.lsst.org/lsst/science/scientist_dark_matter

http://scienceblogs.com/startswithabang/2011/04/20/how-gravi...

http://www.ifa.hawaii.edu/~ger/ASTRO-110_sp08/Lecture28_Dark...