The Big Bang is a theory is and idea that explains how the universe was created. The Big bang happened about 10 billion years ago. Before the big bang the universe was very hot and dence. A big explosion started the expansion of the universe. Since the big bang the universe has been expanding. In the instant—a trillion-trillionth of a second—after the big bang, the universe expanded with incomprehensible speed from its pebble-size origin to astronomical scope. Expansion has apparently continued, but much more slowly, over the ensuing billions of years. Basically the big bang theory setup the universe.
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