A. Merloni

Galaxy clusters are the greatest gravitational ensembles in the Universe. In X-ray glow from hot plasma, the story of cosmic expansion, hidden dark matter, and enigmatic dark energy is encoded.

The night forest is full of fireflies, but before, we only saw those that happened to be in the beam of a flashlight. That was the case with ultraluminous X-ray sources: points in other galaxies that shine in X-rays more brightly than a stellar-mass black hole can steadily emit. Telescopes looked at galaxies chosen for other purposes, missing most of the fireflies.