Observations of the Bullet cluster put limits on the cross-section of the self-interaction of dark matter.

An empirical criticism of MOND, released in August 2006, involves the Bullet cluster, a system of two colliding galaxy clusters.

Specifically, the distribution of dark matter in merging clusters such as the Bullet Cluster shows that dark matter scatters off other dark matter particles only very weakly if at all.

At this point, Gross described the bullet cluster as a clear demonstration of dark matter, while Rubbia referred to data related to the production of matter during the Big Bang; the panel eventually moved on.