A black hole is a region of space where gravity is so strong that nothing, not even light, can escape. This can occur when a star approaches the end of its life. Unable to generate enough heat to maintain its outer layers, it shrinks catastrophically down to an infinitely dense point. When this phenomenon was first proposed in 1916, it defied scientific understanding so much that Albert Einstein dismissed it as too ridiculous to be true. But scientists have since proven otherwise. In 1971, Paul Murdin and Louise Webster discovered the first black hole: Cygnus X-1. Later, in the 1990s, astronomers using NASA's Hubble Space Telescope found that not only do black holes exist, supermassive black holes lie at the heart of almost every galaxy, including our own. On 10 April 2019, a team of astronomers made history by producing the first image of a black hole. This book is the story of black holes.