More than seven billion years ago, two immense black holes circled each other until they collided and merged, a cataclysm so intense that it sent ripples soaring through the fabric of space-time. In the early morning hours of May 21, 2019, Earth trembled from the vibrations sent off by this distant carnage, cluing in astronomers to the biggest cosmic bang they’d ever detected—and one that defies theoretical expectation.

The signal picked up by two observatories—LIGO in the United States and Virgo in Italy— came in the form of gravitational waves: disruptions in space-time that massive cosmic events can set into motion. This signal—named GW190521—came from a truly monstrous collision. Researchers estimate that two black holes 66 and 85 times more massive