Astronomers used the infrared telescope and the old Hubble to discover a gaping void in the center of a galaxy in the Abell 402 cluster, as if an invisible tugboat had pulled billions of stars out of there. The missing mass equals two billion suns: it's not a cloud but a real lack of stars. Near this hole, two powerful sources were found — candidates for supermassive black holes. They orbit at a distance of thousands of light-years and together weigh 60 billion suns — the heaviest pair ever observed. For comparison, the central black hole of the Milky Way is 15,000 times lighter.

The combined gravity of this pair acts like a giant space tugboat: the holes push and eject stars, leaving a swept-clean zone at the center of the galaxy. If the second black hole is confirmed, the system will become the most powerful source of ripples in space-time, which future sensitive detectors could pick up. The first ideas about such waves were developed by Kip Thorne. Remarkably, this pair ejected more stars than shine in some dwarf galaxies: it can accelerate a star to a speed high enough to hurl it forever into intergalactic space.