The Bullet Cluster (1E 0657-558) consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a co-moving radial distance of 1.141 Gpc (3.721 Gly).
Gravitational lensing studies of the Bullet Cluster are claimed to provide the best evidence to date for the existence of dark matter. At a statistical significance of 8σ, it was found that the spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law. However, this interpretation of the gravitational lensing results is disputed, see below. Observations of other galaxy cluster collisions, such as MACS J0025.4-1222, also show significant displacement between their center of visible matter and their gravitational potential.
Read more about Bullet Cluster: Overview, Significance To Dark Matter, Alternative Interpretations
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