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A black hole is an area of spacetime where gravity is so intense that no electromagnetic wave, not even light, has the energy to escape. According to general relativity theory, a compact enough mass may bend spacetime into a black hole. The event horizon is the line beyond which there is no way out. Despite having a significant impact on the outcome and circumstances of an item traversing it, general relativity states that it lacks any locally observable characteristics. A black hole behaves in many ways like a perfect black body since it does not reflect light.Furthermore, event horizons are predicted to produce Hawking radiation with the same spectrum as a black body with a temperature that is inversely proportional to its mass by quantum field theory in curved spacetime. For stellar black holes, this temperature is on the order of billionths of a kelvin, making it virtually hard to view in person.

John Michell and Pierre-Simon Laplace were the first to think about objects whose gravitational fields are so powerful that light cannot escape. Karl Schwarzschild discovered the first contemporary explanation of general relativity that would define a black hole in 1916. The term "black hole" was originally used in 1958 by David Finkelstein to refer to an area of space from which nothing can be ejected. Black holes were once thought to be a mathematical oddity, but theoretical research in the 1960s revealed that they were really a general prediction of general relativity. Jocelyn Bell Burnell's 1967 discovery of neutron stars stimulated research into gravitationally compacted compact objects as a potential astrophysical reality.Cygnus X-1 was the first black hole to be discovered, and it was discovered independently by different astronomers in 1971. Ild discovered the original contemporary answer to general relativity that would define a black hole. The term "black hole" was originally used in 1958 by David Finkelstein to refer to an area of space from which nothing can be ejected. Black holes were once thought to be a mathematical oddity, but theoretical research in the 1960s revealed that they were really a general prediction of general relativity. Jocelyn Bell Burnell's 1967 discovery of neutron stars stimulated research into gravitationally compacted compact objects as a potential astrophysical reality. Cygnus X-1 was the first black hole to be discovered, and it was discovered independently by different astronomers in 1971.

Massive stars collide towards the conclusion of their lives to generate black holes of stellar mass. A black hole can expand after it has created by consuming mass from its surrounds. By consuming more stars and joining forces with other black holes, supermassive black holes with millions of solar masses (M) may arise. The majority of galaxies are thought to contain supermassive black holes.

Through its interactions with other stuff and electromagnetic waves like visible light, black holes can be detected. Any material that falls into a black hole has the potential to create an exterior accretion disk that is heated by friction and gives rise to quasars, some of the brightest objects in the universe. A supermassive black hole may shred stars into streamers that glow brilliantly before being "swallowed" if they pass too close to it. If there are stars circling a black hole, the mass and position of the black hole may be determined from the stars' orbits. Such measurements can be used to rule out potential alternatives like neutron stars.The Sagittarius A* radio source, in the center of the Milky Way galaxy, has a supermassive black hole with an estimated mass of 4.3 million solar masses, according to scientists, who have also detected a number of star black hole candidates in binary systems.

The first direct gravitational wave detection—the first record of a black hole merger—was made public on February 11, 2016, by the Virgo Collaboration and the LIGO Scientific Collaboration. Following studies by the Event Horizon Telescope (EHT) of the supermassive black hole in the galactic center of Messier 87 in 2017, the first direct image of a black hole and its surroundings was released on April 10, 2019. Around 1,500 light-years (460 parsecs) distant as of 2021 is the closest known object that is believed to be a black hole (see list of nearest black holes).There are expected to be hundreds of millions of black holes in the Milky Way, the most of which are solitary and do not produce radiation, even though only a few dozen have been discovered so far. Therefore, gravitational lensing would be the only method for discovering them.