Thousands of galaxies crowd this Webb Space Telescope image of a giant galaxy cluster known as El Gordo--"the fat one." It consists of two large clusters that are merging, about seven billion light-years from Earth. The gravity of the visible galaxies and their dark matter, which is detectable only through its gravitational influence, distort the light of more-distant galaxies, creating long arcs. Astronomers use this lensing effect to calculate the amount of dark matter in the cluster. [NASA/ESA/CSA]