[math-fun] Bullet Cluster considered to be evidence for dark matter

The Bullet Cluster is a pair of galactic clusters that have collided. The gas clouds have interacted, and turned into an X-ray source from shock wave heating.  The majority of the mass, as imaged by gravitational lensing, has passed on through without disturbance.  This object is considered to be evidence for the actual existence of dark matter.   --  Gene