The Coma Cluster contains over a thousand galaxies and has become one of the key pieces of evidence for the existence of dark matter.

The galaxies within it are moving too fast to be held together by visible matter alone.
Furthermore, this cluster
distorts the light of more distant objects—an effect known as gravitational lensing.

This image of the Coma Cluster was captured by the Dark Energy Camera and shows it as a densely populated cluster of entire galaxies.