This image from NASA's Hubble Space Telescope shows the inner region of Abell 1689, an immense cluster of galaxies located 2.2 billion light-years away. The cluster's gravitational field is warping light from background galaxies, causing them to appear as arcs, an effect similar to what appears looking into a fun house mirror.
Dark matter in the cluster, which represents about 80 percent of its mass, is mapped by plotting these arcs. Dark matter cannot be photographed, but its distribution is shown in the blue overlay. The dark matter distribution is then used to better understand the nature of dark energy, a pressure that is accelerating the expansion of the universe. (Photo credit NASA/ESA/JPL-Caltech/Yale/CNRS)