The news: Astronomers have detected the biggest explosion in space that’s ever been observed. The massive eruption occurred in the Ophiuchus cluster, about 390 million light-years from Earth, where thousands of individual galaxies mingle with hot gas and dark matter.

What is it? It is believed to have been caused by a supermassive black hole in the cluster’s central galaxy, according to a new study in The Astrophysical Journal. Black holes sometimes expel huge amounts of energy and matter in the form of two perpendicular jets. This particular event was so big it smashed a gigantic cavity in the gas around the black hole. This cavity is now filled with radio waves created by electrons that the jet had accelerated almost to the speed of light.

How big? To give a sense of the scale of this weird hole in space, you could fit 15 Milky Way galaxies in a row into it, says lead author Simona Giacintucci, director of research at the Naval Research Laboratory in Washington, DC. The explosion is now over, according to the radio data.

How it was discovered: The finding was made thanks to a combination of x-ray data from NASA’s Chandra X-Ray Observatory, Europe’s XMM-Newton space telescope, the Murchison Widefield Array in Australia, and the Giant Metrewave Telescope in India. Researchers noticed unusual activity in the Ophiuchus cluster back in 2016 using data from the Chandra telescope, but the cavity was so large, and would have required so much energy to form, that they initially didn’t think it was feasible.