A black hole is not simply an empty hole in space. It is an extremely compact concentration of mass that creates an enormous curvature of spacetime. The boundary surrounding a black hole is called the event horizon. Once an object passes this boundary, returning to the outside Universe is no longer possible.
Many black holes form when massive stars reach the end of their lives. After a powerful stellar explosion, the remaining core can collapse under its own gravity. If the remaining core is sufficiently massive, the collapse can continue until a black hole forms. Much larger black holes can grow over cosmic time through mergers and the accretion of matter.
The event horizon is the boundary beyond which escape is impossible. It is not a physical surface. Instead, it marks a region where the gravitational geometry of spacetime becomes extreme. For a distant observer, an object approaching the horizon appears to become increasingly slow and faint.
According to classical general relativity, the center of a black hole contains a singularity — a region where the theory predicts extremely high curvature. However, our current theories of physics are incomplete in such extreme conditions. A complete theory of quantum gravity may be required to understand what really happens at the deepest interior.