The Galaxies Bunched to One Side of a Cluster Image Aren't in the Cluster

As NASA describes it, some of the galaxies bunched together on one side of a cluster image are not part of the cluster at all. They sit much farther away, and their light has been bent and magnified by gravitational lensing — which makes apparent neighbours in a photograph into strangers separated by enormous distance along the line of sight. The lens is mass, not glass. Gravitational lensing happens when the enormous mass of a foreground object, such as a galaxy cluster, bends and magnifies light arriving from something much farther behind it. The cluster in front works like a crude magnifying glass, so the background galaxy we see is a bent and enlarged version of the original.