Question: what is a black hole?

Arttu Rajantie answered on 17 Jun 2011:

According to general relativity, mass and energy make spacetime curved. Around a very massive object, the spacetime is so curved that time points inwards. Therefore nothing, even light, can get out, because it would mean travelling backwards in time. Objects like this are called black holes.

In practice, a black hole forms when a large star runs out of hydrogen. The fusion reaction that supports it end, and the star collapses under its own gravitational field and turns into a black hole. We cannot see black holes directly but indirect observations suggest that there are black holes in centres of many galaxies, including our own Milky Way.