A black hole is an object that is so compact (in other words, has enough mass in a small enough volume) that its gravitational force is strong enough to prevent
light or anything else from escaping.
The existence of black holes was first proposed in the 18th century, based on the known laws of gravity. The more massive an object, or the smaller its
size, the larger the gravitational force felt on its surface. John Michell and Pierre-Simon Laplace both independently argued that if an object were either
extremely massive or extremely small, it might not be possible at all to escape its gravity. Even light could be forever captured.
The name "black hole" was introduced by John Archibald Wheeler in 1967. It stuck, and has even become a common term for any type of mysterious
bottomless pit. Physicists and mathematicians have found that space and time near black holes have many unusual properties. Because of this, black holes have
become a favorite topic for science fiction writers. However, black holes are not fiction. They form whenever massive but otherwise normal stars die. We
cannot see black holes, but we can detect material falling into black holes and being attracted by black holes. In this way, astronomers have identified
and measured the mass of many black holes in the Universe through careful observations of the sky. We now know that our Universe is quite literally filled
with billions of black holes.