Everything that exists, including space, time, and matter. The study of the Universe is known as cosmology. Cosmologists distinguish between the Universe with a capital ‘U’, meaning the cosmos and all its contents, and universe with a small ‘u’, which is usually a mathematical model derived from some physical theory. The real Universe consists mostly of apparently empty space, with matter concentrated into galaxies consisting of stars and gas. The Universe is expanding, so the space between galaxies is gradually stretching, causing a cosmological redshift in the light from distant objects. There is now strong evidence that space is filled with unseen dark matter that may have many times the total mass of the visible galaxies; and even more mass may be accounted for by a still-mysterious dark energy. The most favoured concept of the origin of the Universe is the Big Bang theory, according to which the Universe came into being in a hot, dense fireball 13.7 billion years ago.