BEC dark matter can explain collisions of galaxy clusters
  We suggest that the dark matter model based on Bose Einstein condensate or scalar field can resolve the apparently contradictory behaviors of dark matter in the Abell 520 and the Bullet cluster. During a collision of two galaxies in the cluster, if initial kinetic energy of the galaxies is large enough, two dark matter halos pass each other in a soliton-like way as observed in the Bullet cluster. If not, the halos merge due to the tiny repulsive interaction among dark matter particles as observed in the Abell 520. This idea can also explain the origin of the dark galaxy and the galaxy without dark matter. 
Jae-Weon Lee, Sooil Lim, Dale Choi