First off I'm not denying anything. But what I'm having trouble understanding is why it's said that the Big Bang was the beginning of the universe?

"The Universe" is all that exists, and by saying the big bang was the beginning, they would be saying that mater and energy didn't exist before the event. But the event itself was caused by mater and energy. Is there something I'm missing? Does the Big Bang theory claim that there was a point in which mater and energy didn't exist?