General relativity is a theory of space and time. The theory was published by Albert Einstein in 1915. The central idea of general relativity is that space and time are two aspects of spacetime. Spacetime is curved when there is gravity, matter, energy, and momentum.
General relativity has developed into an essential tool in modern astrophysics. It provides the foundation for the current understanding of black holes, regions of space where the gravitational effect is strong enough that even light cannot escape.