Derived from observations of the universe as it is now
Galaxies are moving further away from one another, and the further away they are, the faster they move apart
The speed of expansion makes it possible to work out the age of the universe
According to this theory, about 13.8 billion years ago, there was a sudden burst of energy that marked the creation of time, space, matter and energy
The theory has been supported by the discovery in 1964 of cosmic microwave background radiation and by the abundance of helium and hydrogen in the universe
The most widely accepted version of this theory thinks in terms of one event
Other versions of this theory think in terms of a 'big bang' leading initially to expansion until pulled back by gravity into a 'Big Crunch', after which it all starts again