So, you can't explain why the 'dark matter' is being left behind when galaxy clusters collide. 'Dark Matter Core Defies Explanation in NASA Hubble Image' http://www.nasa.gov/home/hqnews/2012/mar/HQ_12-068_Hubble_Dark_Core.html "This technique revealed the dark matter in Abell 520 had collected into a "dark core," containing far fewer galaxies than would be expected if the dark matter and galaxies were anchored together. Most of the galaxies apparently have sailed far away from the collision. "This result is a puzzle," said astronomer James Jee of the University of California in Davis, lead author of paper about the results available online in The Astrophysical Journal. "Dark matter is not behaving as predicted, and it's not obviously clear what is going on. It is difficult to explain this Hubble observation with the current theories of galaxy formation and dark matter."" The dark matter core does not defy explanation. The dark matter core is not a puzzle. The dark matter core is not difficult to explain. It is obviously clear what is going on. There is nothing to 'leave behind'. Non-baryonic dark matter was never anchored to the matter in the first place. There is no such thing as non-baryonic dark matter. Matter moves through and displaces the aether.