It is a theory of the origin of the universe. According to the big bang theory, the universe began as the result of an explosion that occurred between 15 and 20 billion years ago. Over time, the matter created in the big bang broke apart, forming galaxies, stars, and a group of planets we know as the solar system. The theory was first put forth by Edwin Hubble (1889–1953), who observed that the universe is expanding uniformly and objects that are greater distances are receding at greater velocities. In the 1960s Bell Telephone Laboratories scientists discovered weak radio waves that are believed to be all that remains of the radiation from the original fireball. The discovery further supported Hubble’s theory, which puts the age of the universe between 15 and 20 billion years.

Astronomers have observed that the galaxies are still moving away from each other and that they’ll probably continue to do so forever—or at least for about the next 70 billion years. If the galaxies did come together again, scientists believe that all of the matter in the universe would explode again (in other words, there would be another big bang), and the result would be consistent with that of the first—it would produce a universe much like the one people live in today.

Another supporter of the big bang theory is British theoretical scientist Stephen Hawking (1942-). In 1988 Hawking, who is known for his theories on black holes (gravitational forces in space, made by what were once stars), published his ideas in the best-selling book A Brief History of Time: From the Big Bang to Black Holes.