A black hole is a region of space in which gravity is so overwhelming that nothing, not even light, can escape once it crosses a boundary called the event horizon. The concept was first suggested as a mathematical curiosity in the late 18th century, but it became a serious part of physics only after Einstein's general theory of relativity, published in 1915, provided the equations that describe how mass warps the geometry of space and time. For decades afterwards, even most physicists regarded black holes as an abstraction - an inevitable consequence of the mathematics that was unlikely to correspond to any real object.
Indirect evidence began to accumulate in the second half of the 20th century. Astronomers noticed stars orbiting invisible companions whose masses could be calculated from their gravitational effects, and sources of intense X-ray radiation that matched theoretical predictions for matter heated as it fell towards a black hole. By the 1990s, a supermassive object at the centre of the Milky Way had been mapped in such detail that no plausible alternative to a black hole remained.
The first direct image came only in 2019, produced by the Event Horizon Telescope, an international collaboration that effectively turned the entire planet into a single radio dish. The image showed a dark central region surrounded by a glowing ring of hot gas, exactly as relativity had predicted. A second image, of the Milky Way's own central black hole, followed in 2022.
Black holes continue to challenge the assumptions of physics. Quantum mechanics and general relativity, the two most successful theories of the 20th century, give contradictory answers when applied together to the region near an event horizon. Resolving this conflict, sometimes called the black hole information paradox, is widely regarded as one of the most important open questions in physics. The answer, when it comes, is expected to reshape our understanding not only of black holes but of space and time themselves.