According to the Big Bang Theory, about 13.8 billion years ago the whole Universe was a very small, extremely hot and dense region. From this tiny point, the whole Universe expanded outwards to what exists today.




Astronomers have discovered that, in general, the further away a galaxy is, the more red-shifted its light is. This means that the further away the galaxies are, the faster they are moving. This is similar to an explosion, where the bits moving fastest travel furthest from the explosion. 


Red-shift data provides evidence that the Universe, including space itself, is expanding.