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. This separation is powerful evidence for a component that interacts much less strongly than hot plasma.

Ordinary plasma particles collide electromagnetically, so the gas is displaced from the collisionless components. If most gravitating mass consists of weakly interacting dark matter, its distribution should remain closer to the collisionless galaxies than to the slowed gas.

Chandra X-ray observations locate the hot gas, while Hubble and ground-based lensing measurements locate the dominant mass peaks separately. Similar systems provide additional collision tests, though detailed modeling is important.

X-ray plasma contains much of the baryonic mass, while collisionless galaxies pass through more freely. Lensing maps show major mass peaks closer to the galaxy distributions than to the shocked gas.