Big Bang Theory:
The widely accepted theory of the creation of the universe is the Big Bang Theory. In 1948, George Gamow proposed that the universe was created in a gigantic explosion of the primeval atom and that the various elements observed today were produced within the first few minutes after the big bang. At that time, the temperature that ranged somewhere near 10 billion degrees helped fuse subatomic particles into what we now know of as the chemical elements.
The extremly high density within the primeval atom would cause the universe to expand at near light-speed. As this matter expanded, the hydrogen and helium would cool and condense into what would later become the stars. This would explain the expansion of the universe and the physical basis of Hubble's law. The original explosion propelled them away from each other, and they have continued doing so, all the while slowing down.
This presents some interesting possibilities. When the velocity of the galaxies reaches zero, will they start pulling in towards the center of the universe, to once again combine into the primeval atom, where it will then explode again? Is our universe then only one in an endless chain of universes? Or will the galaxies continue to move away from each other forever? Might they stop moving after a while, and just stay motionless? These questions and more have to be answered before the Big Bang Theory can be accepted as the truth, but we may never truly know for sure.
Creation of a Cosmology: Big Bang Theory
Microsoft Encarta 1995
The Macmillian Book of Astronomy