A black hole is a region in space with such a strong gravitational pull that even light cannot escape from it. This is caused by a very dense and compact mass, usually formed from the collapse of a massive star.
The mathematical theory of black holes is based on Albert Einstein's theory of general relativity. This theory describes how gravity works and explains the curvature of space and time around objects with large masses, such as black holes.
The size of a black hole is determined by its event horizon, which is the point of no return for anything that gets too close to the black hole's gravitational pull. The size of the event horizon depends on the mass of the black hole.
No, once something crosses the event horizon of a black hole, it is impossible for it to escape. This is because the gravitational pull of the black hole is so strong that even light cannot escape from it.
At the center of a black hole lies the singularity, a point where the gravitational pull becomes infinite and the laws of physics break down. It is believed that the singularity is a point of infinite density and zero volume.