a group of objects that are in the dark

Wide Field Image of Galaxy Cluster SDSS J1110+6459 Caption: The galaxy cluster shown here, SDSS J1110+6459, was discovered as part of the Sloan Giant Arcs Survey. It is located about 6 billion light-years from Earth (redshift of z=0.659) and contains hundreds of galaxies. At left, a distinctive blue arc is actually composed of three separate images of a more distant background galaxy called SGAS J111020.0+645950.8. Credits: NASA, ESA, and T. Johnson (University of Michigan)