Black hole is a region of space whose gravitational force is so strong that nothing can escape from it. A black hole is invisible because it even traps light. The gravitational force is so strong very near the black hole because all its matter is located at a single point in its center. Physicists call this point a singularity. It is believed to be far smaller in size than the nucleus of an atom. Most astronomers believe that the Milky Way galaxy--the galaxy in which Earth is located--contains millions of black holes. No one has discovered a black hole for certain. However, astronomers have found strong evidence for seven black holes in the Milky Way and in a nearby galaxy.