What is a black hole?
One-line takeaway
BLUF: A black hole is a region of spacetime with gravity so intense that nothing – not even light – can escape from it.
A fundamental explanation of what is a black hole?
The Explanation
A black hole is a region of spacetime with gravity so intense that nothing – not even light – can escape from it. It forms typically when a massive star collapses, compressing mass into a very small volume. The boundary of a black hole is the event horizon, inside which escape velocity exceeds the speed of light. At the center lies a singularity (in classical GR) where density is infinite. Practically, a black hole's gravity warps space intensely; any object crossing the horizon is lost to the outside universe. Astronomers detect black holes by their effects on nearby matter (accretion disks, gravitational waves) rather than seeing them directly.