Who: Karl Schwarzchild, modern interpretation David Finkelstein.
What: A place where ordinary gravity is so extreme it prevents anything from escaping.
Where: Outer space. There is a huge one at the centre of the Milky Way, our galaxy, 4.3 million times as massive as the Sun.
Why: Formed when giant stars collapse in on themselves at the end of their life cycle.
When: First hypothesized in 1783, scientists are still theorizing them.
“I used to think information was destroyed in black hole. This was my biggest blunder, or at least my biggest blunder in science.” – Stephen Hawking
Black holes are essentially the vacuum-cleaner remains of dead stars. They can be big or small, but are characteristically massive – a black hole the size of an atom would have the mass of a large mountain. What makes black holes interesting is they don’t even let light escape, creating distortions in space and time. If you dropped a clock into a black hole, it would slow down and eventually stop – but if you fell in with the clock, it would not appear to change at all. And some scientists think it’s possible that black holes form wormholes to other points in space, or even other universes, although you would have to survive the mind-numbingly inhospitable conditions of a black hole (gravity pulling you apart, x-rays frying you alive), and then escape from the black hole on the other side.