The Bullet Cluster is made up of two colliding clusters galaxies. According to a 2006 study, the Bullet Cluster also shows the best evidence for the existence of Dark Matter. From observations of galaxy cluster collisions it has been found that many show displacement between their center of visible matter and their gravitational potential.
Each component, stars, gas, and dark matter, within a cluster pair behaves differently during a collision allowing for each to be studied separately. The mass of the ordinary matter of the cluster pair is made up primarily from the hot gas of the two colliding cluster pairs. The electromagnetic interaction of the gases causes them to slow much more than the stars within the cluster pair. The stars see little effect from the collision and usually pass through with some gravitational slowing but otherwise unaltered. Dark matter, the third major component has been indirectly detected by the gravitational lensing of background objects. The fact that the lensing is strongest in the two separated regions near the visible galaxies lends support to the concept that most of the mass in the cluster pair is in the form of collisionless dark matter.
The Bullet Cluster is one of the hottest known clusters of galaxies. It derives it’s name from the fact that the smaller subcluster is moving away from the larger. From what we can see on Earth, the subcluster went through the cluster center 150 million years ago.
Certain authors report that the cluster is undergoing a high-velocity (around 4500 km/s) merger. The spatial distribution of the X-Ray emitting gas provides evidence of this; however, this gas lags behind the subcluster galaxies. The dark matter mass is coincident with the collisionless galaxies, but is in front of the collisional gas.
In recent studies of the cluster MACS J0025.4-1222 there are indications that a titanic collision has separated the dark from ordinary matter.
Due to the Bullet Cluster phenomenon we have evidence that dark matter exists on large cluster scales, it offers no specific insight into the original galaxy rotation problem. the observed ratio of visible matter to dark matter in a typical rich galaxy cluster is much lower than predicted.