A huge cluster of galaxies lies in the northern constellation of Perseus. At a distance of about 300 million light-years away, it is one of the nearest galaxy clusters. The entire cluster contains thousands of galaxies and is considered to be one of the most massive objects in the universe. Astronomers believe the cluster's smallest galaxies have survived intact for billions of years. Hubble has photographed four of these dwarf galaxies, which are thought to be embedded in halos of an invisible form of matter called dark matter.