The Big Bang theory states that the universe began with an instantaneously expanding point, about 10 - 20 billion years ago.

What are the foundations that the theory rests on?

Einstein's General Theory of Relativity (which describes how the distribution of mass in the universe determines the geometry of the space), and the observation that the distribution of matter in the universe, on a large scale, is nearly uniform.

Is the universe really expanding?

Edwin Hubble, in the 1920s, proved that other galaxies are moving away from our own, and that more distant galaxies are moving away from us at a faster speed than those close to us. An expanding universe 'stretches' light rays to create a 'red shift' effect, explaining why more distant galaxies appear redder.

Does the expansion of the universe prove the Big Bang?

Not by itself, although it suggests that there was a point when the universe started to expand. Other proofs of the Big Bang come from the observed abundances of helium, deuterium and lithium, and the thermal spectrum of the background radiation.

How does the amount of lithium, helium and deuterium in the universe prove the Big Bang theory?

Big Bang theory states that the early universe was extremely hot, and consisted of a sea of photons, neutrons, protons, positrons, electrons, and neutrinos (the particles which atoms are made up out of). As the universe began to cool, the neutrons either decayed into protons and electrons, or combined with other sub-atomic particles to make deuterium. Most of the deuterium then combined to make helium (some lithium was also produced). Elements heavier than lithium are synthesised in stars. The Big Bang theory predicts the amounts of lithium, deuterium and helium in the universe based on the amounts that were calculated to have been formed during the first few minutes of the universe.

How does the thermal spectrum of background radiation prove the Big Bang?

The Big Bang theory predicts that the universe was extremely hot (about 10 billion degrees), and has cooled since. Background radiation appears hotter in more distant clouds of gas. Since light travels at a finite speed, it takes time to reach us from these distant clouds, and instead of seeing what the distant clouds are like now, we see them as they were at an earlier time in the history of the universe, when it was hotter.

What doesn't the Big Bang theory explain?

It doesn't explain the origin of galaxies, the large-scale clustering of galaxies, and the origin of the uniform distribution of matter on extremely large scales. Cosmologists think that the Inflation theory, an extension on the Big Bang theory, may explain these.