Black holes are regions where gravity is so intense that nothing—not even light—can escape. First predicted by Einstein's general relativity, they were long considered theoretical curiosities before observational evidence confirmed their existence.
Black holes form when massive stars exhaust their nuclear fuel and collapse. If the remaining core exceeds about three solar masses, no known force can halt the collapse. The matter compresses to a singularity—a point of theoretically infinite density.
The "event horizon" marks the point of no return. Beyond it, escape velocity exceeds the speed of light. From outside, nothing that crosses this boundary can ever be observed again.