Out of the ancient cosmic darkness swirled the impossible. That is one way to describe the biggest recorded collision of two black holes, a cataclysmic event detected in May last year but revealed by scientists only last week.

The distant merger, 17bn light years away, revealed itself through the gravitational waves that rippled outwards from this violent union, which happened around 7bn years ago and took just a fraction of a second. The waves — wobbles in the space-time fabric of the universe — were detected by the Ligo (laser interferometer gravitational wave) and Virgo observatories based in the US and Italy respectively. These detectors are designed to pick up gravitational ripples induced by the most dramatic events in the universe, such as black holes devouring each other or huge stars exploding.  

The event, named GW190521, challenges our understanding of these extreme objects. So-called stellar black holes are the corpses of stars; when massive stars run out of fuel, they collapse in on themselves. In some cases, this contraction creates a core so dense that nothing, not even light, can escape its gravitational grasp. Our own sun, whose solar mass provides the benchmark for star sizes, is not massive enough to become a black hole in its dotage.