In 1927 a Roman Catholic priest and scientist Georges Lemaitre proposed what later became known as the Big Bang theory of the origin of the Universe, based on work by Edwin Hubble who theorized and then proved that the Universe was getting bigger and bigger. Many scientists have discovered other evidence that confirms that the theory might be correct. They think that the Big Bang happened 13 to 15 billion years ago.
Big Bang theorists are called cosmologists, because they study the cosmos. They tell us that before the big bang, the entire Universe fit into a space that would make a grain of sand absolutely colossal. Everything that exists, from a blade of grass to Sirius, the Dog Star, all fit into a very, very tiny space, all compacted together.
Suddenly there was an explosion, and the Universe began to spread out. Expanding at the speed of light, which is 186,000 miles per second, the Universe continues to expand today, and no one is sure when it might end, but it’s estimated to be billions of years in the future.
The Big Bang theory does not explain how the Universe began, or where all of the “stuff” in it comes from, or how it was created. Scientists are certain that the Universe has a beginning, but are not certain what that was. Science does not and cannot explain or describe “the beginning,” only the general evolution of our Universe from a possible point in time.