Bullet Cluster

The Bullet Cluster is a collision between two galaxy clusters in which hot gas and most gravitationally inferred mass occupy different locations. The separation is a prominent line of evidence for dark matter.

Galaxy cluster collision: An encounter in which galaxy clusters pass through one another under gravity. The Bullet Cluster is a particularly well-observed example of this large-scale collision.
Musket Ball Cluster: A pair of colliding galaxy clusters observed in a later merger system. It provides another system in which X-ray gas and lensing mass are spatially offset.
Modified Newtonian dynamics: A proposal that alters Newtonian gravity at very low accelerations to explain galactic dynamics without particle dark matter. The Bullet Cluster challenges versions of modified gravity that lack an additional unseen mass component.
Dark matter evidence: Observations whose gravitational or cosmological effects imply matter beyond directly observed luminous matter. The Bullet Cluster is a striking, though not standalone, piece of this broader evidence.
Intracluster medium: The thin, hot plasma that fills the space between galaxies in a galaxy cluster. Collisions make this gas interact and slow, leaving it displaced from the lensing mass.
Abell 520: A merging galaxy cluster whose lensing mass distribution has been interpreted as unusual relative to its galaxies and gas. Its contested central mass concentration complicates simple comparisons with the Bullet Cluster.
Self-interacting dark matter: Dark matter models in which dark matter particles scatter off one another. The observed mass–galaxy alignment constrains how strongly dark matter can interact during collisions.
Cosmic microwave background: Relic radiation released when the early universe became transparent to light. Its measured structure independently supports a universe containing nonbaryonic dark matter.
Gravitational lensing: The bending of light by matter and energy along its path. Lensing maps the cluster’s gravitational mass, including matter that emits little or no light.
MACS J0025.4-1222: A merging galaxy cluster system in which lensing mass is offset from X-ray-emitting gas. It offers an independent example of mass–gas separation in a cluster collision.

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