What is a black hole and how does it form?

A black hole is a region of space where gravity is so intense that nothing, not even light, can escape once it crosses the boundary called the event horizon. This extreme gravity results from an enormous amount of mass compressed into a very small space. Most black holes form when a massive star, typically more than 20 times the mass of our Sun, exhausts its nuclear fuel and can no longer support itself against gravity. The core collapses catastrophically in a supernova explosion, and if the remaining mass is sufficient, it continues collapsing into a singularity, a point of infinite density. Supermassive black holes found at galaxy centers likely formed through mergers and accretion over billions of years, though their exact origins are still studied.