What observation is central to the study of the Bullet Cluster?

The study of the Bullet Cluster is primarily centered on the interplay of dark matter and gas. The Bullet Cluster is a collision of two galaxy clusters that offers crucial evidence for the existence of dark matter. In this event, the two clusters passed through each other without significantly interacting, while the hot gas contained in the clusters experienced friction and slowed down.

Observations show that the visible matter (in the form of gas) is separated from the gravitational mass, which is inferred from the lensing effect on background galaxies. Observations of gravitational lensing allow astronomers to map the distribution of this unseen mass, revealing that the majority of the mass does not coincide with the visible hot gas. This discrepancy indicates that a significant portion of the cluster's mass must be in the form of dark matter, which does not interact electromagnetically and thus remains unaffected during the collision.

This phenomenon provides strong evidence for dark matter's existence and behavior under extreme conditions, highlighting how it influences structure formation and dynamics in the universe.