Please explain the Big Bang theory as simply as possible.
The Big Bang Theory is an explanation for the creation of the universe. By calculating backwards using the General Theory of Relativity, we find that the universe has been expanding for a long time. For something to expand, it must have a starting point to expand from. According to General Relativity, energy cannot be created or destroyed; it is all here and measurable in some form. Therefore, there must have been a single place where all the existing energy in the universe was compressed. When this point of energy popped, or "banged," it started to expand, allowing energy to form matter, eventually creating galaxies and solar systems.
In 1930, Georges Lemaître, a Catholic Priest and physicist, condensed existing science into a theory he originally called "the Cosmic Egg exploding at the moment of creation." His theory is still the center of Big Bang science studied today.