In 1929, in the California Mount Wilson observatory, an American astronomer by the name of Edwin Hubble made one of the greatest discoveries in the history of astronomy. While he observed the stars with a giant telescope, he found out that the light fromthem was shifted to the red end of the spectrum and that this shift was more pronounced the further a star was from the earth. This discovery had an electrifying effect in the world of science, because according to the recognised rules of physics, the spectra of light beams travelling towards the point of observation tend towards violet while the spectra of the light beams moving away from the point of observation tend towards red. During Hubble's observations, the light from stars was discovered to tend towards red. This meant that they were constantly moving away from us.

Before long, Hubble made another very important discovery: Stars and galaxies moved away not only from us, but also from one another. The only conclusion that could be derived from a universe where everything moves away from everything else is that the universe constantly 'expands'.

To better understand, the universe can be thought of as the surface of a balloon being blown up. Just as the points on the surface of a balloon move apart from each other as the balloon is inflated, so do the objects in space move apart from each other as the universe keeps expanding.