A singularity is the point at the center of a black hole where, according to general relativity, matter is crushed to infinite density and spacetime curvature becomes infinite. It marks the place where the known laws of physics break down, because a complete description would require a theory of quantum gravity that does not yet exist. Singularities are hidden from view behind the event horizon, so they cannot be observed directly. Many physicists regard the infinities as a sign that general relativity is incomplete rather than a literal description of nature.