The Big Bang brought our universe into existence between ten and twenty billion years ago. Why this happened remains a great mystery. It is, however, relatively certain that this Big Bang actually did take place.
The Big Bang theory is a physical model for the creation of the universe, which states that time, space and matter appeared simultaneously. This occurred in a gigantic explosion. During the first minutes or even fractions of a second hydrogen and helium were formed. In this moment, the temperatures must have been a billion, multiplied by a billion, multiplied by a billion degrees Celsius, which, in the next fraction of a second, cooled down to one billion degrees.
Initially, the young universe must have been a glowing fireball. Because of increasing expansion, hydrogen and helium cooled down and became denser, giving rise to galaxies and stars. However, very little is yet known about this stage. It is thought that it did not take place until about one billion years after the Big Bang.
The universe is still expanding today, with the galaxies drifting apart as after a great explosion. This is seen as an indication that the Big Bang actually happened. The further the galaxies are apart, the faster the distance between them grows.
Various theories exist on the further development of our universe. Besides the one described above, which expects a continuing expansion of the universe, another states that the expansion will actually end at some point. This theory even expects that gravity will eventually pull the galaxies back together, faster and faster, until they are destroyed in a new Big Bang.
The more likely theory proclaims that the universe will continue to expand, as the total mass of all galaxies is not sufficient to act as a gravitational "brake". Unless the universe contains large amounts of dark mass that could cause this, the theory of the ever-expanding universe seems to be the more likely one.
