Anyone hauling a boulder to the top of a skyscraper and dropping it on to the street below would expect a catastrophic impact: flying shards of rock and road, streaks of sparks and smoke, the clap and crack of the reverberating bang. Similarly, when a planet, comet or any other material falls into a star, the resulting explosion is often dramatic. It can produce blinding flashes, bursts of high-energy X-rays and gigantic glowing flares of fiery gases.

But, in early 2001, astronomers observing a strange object 6,000 light-years from Earth with the orbiting Hubble Telescope reported that they had seen the opposite. Massive clumps of hot gases many times larger than Earth were being sucked down into a large, invisible object. As they raced downwards, the accelerated jostling heated them, and they glowed and pulsed with incredible energies. Then nothing. The gases just disappeared. There was no explosion, no flashes, no flares – just nothing.

Einstein's theory of general relativity predicts the existence of black holes: bodies so dense that their gravity captures everything close by and prevents it from escaping. Not even light, the fastest and most nimble signal known, can climb up and away from a black hole. They are colourless, invisible, wholly black patches in the sky. For this reason, physicists doubted for many years that black holes could ever be seen at all. And if they could not be observed, they doubted they were a serious part of empirical science.