One theory that accounts for the universal expansion is the big bang--- the idea that the universe began at a specific moment in time and has been expanding ever since. Evidence from the cosmic background microwave background radiation and the relative abundance of light elements, in addition and the expansion, support the big bang theory.
At the moment of creation, all forces and matter were unified in one unimaginably hot dense volume. As the universe expanded, however, a series of six"freezings" led to the universe we see today. Freezings at 10-43 second, 10-35 second, and 10-10 second caused a single unified force to split into the four forces we observe today: gravitational, strong, electromagnetic, and weak forces. At that early stage of the universe, when all matter and energy were contained in a volume no larger than a grapefruit, matter was in its most elementary from of quarks and leptons.
At 10-5 second, the quarks bonded together to form heavy nuclear particles such as protons and neutrons. Subsequent freezings saw these particles first fuel to nuclei at three minutes, and ultimately join with electrons to from atoms at 500,000 years. Stars, which formed from those atoms, then could begin the processes that provided all other chemical elements.
The search for dark matter--- mass that we cannot see with our telescopes--is a research frontier that may help us determine whether or not the universe will continue expanding forever.
NOW LET US GO OVER IN DETAIL AND SEE HOW SCIENTISTS LOOK AT THE EVOLUTION OF OUR UNIVERSE!
