... Many are images of a single, unusual, beaded, blue, ring-like galaxy which just happens to line-up behind a giant cluster of galaxies. Cluster galaxies here appear yellow and -- together with the cluster's dark matter -- act as a gravitational lens . ... Publications with keywords: cluster of galaxies - cluster - gravitational lens . Publications with words: cluster of galaxies - cluster - gravitational lens . ... The Hercules Cluster of Galaxies . ... The Einstein Cross Gravitational Lens . ...