El Gordo (lit. The Fat One) (ACT-CL J0102-4915 or SPT-CL J0102-4915) is the largest distant galaxy cluster observed at its distance or beyond, as of 2011. As of 2014, it still holds the record for being the largest distant galaxy cluster to have been discovered with a mass of 3 quadrillion suns.^[3][4][5][6] It was found by NASA's Chandra X-ray Observatory, Atacama Cosmology Telescope - funded by National Science Foundation, and European Southern Observatory's Very Large Telescope.^[7]

This galaxy cluster, officially named as, 'ACT-CL J0102-4915', has been given a 'nickname' by the researchers as 'El Gordo', which stands for "the Fat One" or "the Big One" in Spanish. It is located more than 7 billion light-years from Earth.^[8]

Findings and results on 'El Gordo' were announced at the 219th meeting of American Astronomical Society in Austin, Texas.^[9]

Felipe Menanteau (then of Rutgers University) who led the study stated "this cluster is the most massive, the hottest, and gives off the most X-rays of any known cluster at this distance or beyond."^[10]

Findings from the European Southern Observatory's Very Large Telescope and the Chandra X-ray Observatory show that El Gordo is composed of two separate galaxy subclusters, colliding at several million kilometers per hour.^[11] These observations (using X-ray data and other characteristics) suggest that 'El Gordo' most probably formed in the same manner as the Bullet Cluster (which is located 4 billion light years from Earth).^{[12][13][14][15]} According to Cristóbal Sifón from Pontifical Catholic University of Chile "this is the first time we've found a system like the Bullet Cluster at such a large distance."^[16]