Schematic diagram showing the expansion of the Universe. The Universe was born in a cataclysmic even called the Big Bang, 13.7 billion years ago. Since then, it has been expanding, increasing in size at an ever faster rate. As it expands, galaxies are pulled apart. The cosmic background radiation - the 'afterglow' of the Big Bang which even now pervades the Universe - is not immune to the expansion either. Its wavelength, as indicated by the since waves at the bottom, has gradually increased with time, now filling spacetime with microwave energy.