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.""
You cannot explain what is happening. You can only give an unevidenced illogical pseudo-science guess about some mythical aether with magical proporties.
'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.""
You cannot explain what is happening. You can only give an unevidenced illogical pseudo-science guess about some mythical aether with magical proporties.