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A galaxy cluster behemoth
Galaxy clusters form through mergers of smaller groups of galaxies. These events depend on the amount of dark matter and dark energy in the universe, and thus could shed light on these enigmas.

Dark energy seems to make up 73 percent of all the mass and energy in the universe, and is driving the accelerating expansion of the universe. Unseen and as-yet-unidentified dark matter makes up about 23 percent of all the matter and energy in the universe — scientists know it exists because of the gravitational effects it has on galaxies. The regular matter that makes up humans, planets and stars constitutes only 4 percent of the universe.

El Gordo was discovered using NASA's Chandra X-ray Observatory in space and the Atacama Cosmology Telescope in Chile. Its Spanish nickname is a nod to the Chilean connection.

The scientists detailed their findings Tuesday at the annual meeting of the American Astronomical Society here during a presentation that included a separate announcement of the discovery of the most distant galaxy cluster ever seen in the early universe.