The Big Bang: Origin of the Universe
Before the universe as we now know it existed, there was no space or time. The Big Bang and its associated theories try to explain or describe the moment of change from nothingness and no time to the existence of the universe filled with space and marked by time. Many physicists describe this event as an explosion, or flash, hence the name Big Bang. The Big Bang is a process of expansion in our universe that is still active today.
The universe flashed into existence (according to the Big Bang theory) from a very small agglomeration of matter of extremely high density and temperatures. As a dense, hot globule of gas, containing nothing but hydrogen and a small amount of helium, it began expanding rapidly outward. There were no stars or planets. The first stars probably formed when the universe was about 200 million years old. Our Sun was formed 4.5 billion years ago, and through telescopes we can now see stars forming out of compressed pockets of hydrogen in outer space.
In 1992 instruments aboard the Cosmic Background Explorer (COBE) satellite, launched in 1989, showed that 99.97% of the radiant energy of the universe was released within the first year of the Big Bang event. This evidence seems to confirm the Big Bang theory. In March 1995 astronomers found more supporting evidence for the big bang when they concluded that data obtained from the space shuttle's Astro 2 observatory showed that helium was widespread in the early universe. The Big Bang theory holds that hydrogen and helium were the first elements created when the universe was formed.