The Big Bang theory is an attempt to deal with the observed reality of an expanding universe. That is to say, in the wake of observations showing that all galaxies are moving away from one another, the "film" was run backward mentally, projecting back to a time when all the matter and energy in the universe was compressed into a single unit. In fact, this was a time before matter and energy had differentiated. This massive block of substance then exploded outward in all direction, differentiating into matter and energy, and into the different nuclear forces. That explosion is the Big Bang.
The big bang theory is the explanation most commonly accepted by astronomers (scientists specializing in the study of matter in outer space) for the origin of the universe. According to this theory, the universe began 15,000,000,000 to 20,000,000,000 years ago as a single point that underwent a tremendous explosion, called the "big bang." Particles were spewed forth and became the building blocks of matter (anything that has a form and occupies space). In time, this matter came together in huge clumps to form galaxies (groups of bodies). Smaller clumps within the galaxies formed stars. Parts of at least one clump of matter became the planets in Earth's solar system (the group of bodies that revolve around the Sun).
The big bang theory was first proposed in 1929 by American astronomer Edwin Hubble (1889–1953), who demonstrated that the universe is still expanding. Soon thereafter Belgian astronomer Georges-Henri Lemaître (1894–1966; known as "Father of the big bang") determined that if the universe is expanding, then by going back in time scientists would find that everything began at a single point.
The big bang theory received a boost in 1948, when Russian-born American physicist (a scientist specializing in the interaction between matter and energy) George Gamow (1904–1968) deduced that if a bang had occurred, it would have left traces of background radiation (energy released by atoms in space). By the time of the current era, that radiation would have cooled to just a few degrees above absolute zero (the lowest possible temperature at which matter can exist, equal to -459°F (-273°C). Gamow's theory was confirmed in the mid-1960s by radio engineers Arno A. Penzias (1933– ) and Robert W. Wilson (1936– ), who worked at Bell Telephone Laboratories. They discovered weak radio (radiation) waves that are believed to be the remains of the original big bang fireball. Another important supporter of the big bang theory is British physicist and mathematician Stephen Hawking (1942– ), who has done extensive work on black holes. A black hole is an area in space created by a collapsed star; the material is so dense that nothing—not even light—can escape. Hawking's best-selling book, A Brief History of Time: From the Big Bang to Black Holes (1988), has increased understanding and support for the big bang theory.
Astronomers have observed that galaxies are still moving away from each other and that they will probably continue to do so for at least another 70,000,000,000 years, if not forever. If the galaxies were to come together again, scientists believe the result would be another big bang, producing a universe much like the one we have now.
Further Information: Big Bang Theory. [Online] Available http://map.gsfc.nasa.gov/html/big_bang.html, November 8, 2000; "Big Bang Theory." MSN Encarta. [Online] Available http://www.encarta.msn.com/find/search.asp?search=big+bang+theory, November 8, 2000; Silk, Joseph. The Big Bang. New York: W. H. Freeman, 1988.
In the "beginning" of the Universe, there was no Universe. All of the matter that we know exists today was compacted into an incredibly small space. As you know, the greater the amount of mass in a unit of space or point, the denser the object or point is. This "point" that held all of the matter in the universe was the densest object that we have ever discovered. Anyway, this point exploded and flung out all of its matter into space. This is the Big Bang Theory: the theory that a big bang (the explosion) caused the Universe to form. Edwin Hubble is credited with coming up with this theory. He observed that all objects in the Universe, especially galaxies, are continually moving away from each other. He also discovered that a galaxy's velocity is proportional to its distance. For more information this and the Big Bang Theory in general, take a look at the website that I have provided.