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.
"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
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