A black hole is a spot in space where gravity is so strong that nothing, not even light, can escape. They are formed when a very large star (much larger than our sun) runs out of fuel and collapses in on itself. The entire mass of the star is compacted into a very tight space, so a black hole’s density is enormous.

Nearly all galaxies are believed to have a black hole in their centers, including our own Milky Way Galaxy!