A gravitational lens is a massive celestial object whose gravitational forces bend and reflect light that passes by, causing background objects to appear distorted. The ghostly “ring” of dark in the galaxy cluster Cl 0024+17, 2.6 million light years across, is one of the strongest pieces of evidence for dark matter. Astronomers believe that the ring was created when two galaxy clusters collided and left a ripple of dark matter, which now warps the light of distant galaxies behind it. The second image, produced by the Large Synoptic Survey Telescope, depicts the mass distribution in this same cluster. It basically shows how normal graph paper would be distorted if it was placed behind this particular gravitational lens, and therefore demonstrates how much these distant galaxies are actually distorted. The LSST aims to reconstruct the detailed mass distribution of the universe, which will help scientists understand the nature and distribution of dark matter, and its role in galaxy formation.