The Bullet Cluster is a specific pair of galaxy clusters
about 1.141 Gpc distant from us, which appear to have
collided with each other.
The name actually refers to the smaller of the two.
The intracluster medium (ICM) (plasma) looks as if it were combined by the
apparent collision, but surrounding gravitational lensing is
consistent with dark matter that passed through as though
unaffected by electromagnetic effects.
This is inconsistent with the alternatives to dark matter that have
been theorized, such as MOND, which explains the dynamics of
galaxies and galaxy clusters by positing the effect of
a weak gravitational force differs from that implied by Newton's laws,
rather than by presuming some hidden mass.