A black hole is a region of spacetime where gravity is so strong that nothing can escape it, including light. It can be formed when a mass of sufficient size distorts spacetime. This distortion creates an event horizon, a boundary of no return where any object that crosses it will be significantly affected by the black hole, reflecting no light.
The phenomena of black holes were first theorized in the 18th century by John Michell and Pierre-Simon Laplace, who suggested the existence of objects with gravitational fields so powerful that light couldn't escape. A century later, in 1916, Karl Schwarzschild found the first modern solution to the concept of a black hole according to the theory of general relativity. But it wasn't until David Finkelstein's 1958 interpretation of a "black hole" as a region from which nothing can escape, that the concept was truly solidified.
Black holes can form at the end of a star's lifecycle, particularly massive stars. Once formed, black holes can accrue mass from their surroundings, and supermassive black holes can develop from the absorption of other stars or the merging with other black holes. These supermassive black holes are thought to exist in the centers of most galaxies.
The presence of a black hole can't be seen directly, but it can be inferred from its interaction with other matter and with electromagnetic radiation such as visible light. When matter falls onto a black hole, it can form an external accretion disk that can form quasars, some of the brightest objects in the universe. Observing the gravitational effects on objects near a black hole can help determine the mass and location of the black hole.
The concept of a space body so large that light cannot escape was initially proposed by John Michell in 1784. Michell's calculations assumed that such a body might have similar density to the sun and a surface escape velocity exceeding normal light speed. These invisible 'dark stars' could theoretically be detectable through their gravitational effects on nearby visible bodies. However, this theory lost traction when the wave-like nature of light was discovered in the early 19th century.
The current understanding of black holes is based on the modern theory of gravity, general relativity, which suggests that light does not leave the surface of a black hole due to the curvature of spacetime itself.