The Bullet Cluster & Collision Evidence: Structures & Vocabulary

The Bullet Cluster is a collision between two galaxy clusters in which most ordinary baryonic mass, observed as X-ray-emitting gas, is spatially separated from most of the gravitationally inferred mass mapped by lensing. Cluster collisions constrain dark-matter self-interaction and challenge simple explanations in which the apparent mass discrepancy follows only the dominant baryonic gas distribution.