A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no light can escape is called the event horizon. Although crossing the event horizon has huge effects on the fate of an object, it appears to have no locally detectable features. In many ways, a black hole acts like an ideal black body, absorbing all incoming radiation and reflecting none.
Most black holes are found in binary systems: two stars orbiting each other so closely that their mutual gravitational attraction causes one or both of them to collapse into an extremely dense object (compared to its original size), leaving behind a “black hole” whose gravity prevents anything else from getting close enough to be affected by its immense gravitational field.