... Dark Matter Ring Modeled around Galaxy Cluster CL0024 17 . ... These are multiple images of a few distant galaxies, showing that the cluster is a strong gravitational lens . ... A hypothesis for the formation of the huge dark matter ring holds that it is a transient feature formed when galaxy cluster CL0024+17 collided with another cluster of galaxies about one billion years ago, leaving a ring similar to when a rock is thrown in a pond. ... Abell 2218: A Galaxy Cluster Lens . ...