Well the proof is that we have energy now. By the laws of conservation, the Big Bang could not have created energy, thus energy was present at the time the Big Bang occurs, thus the Big Bang is not the beginning of the universe, but rather the beginning of observable time.
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Your statement would be correct in that case, but I was taught that it [b]was[/b] the beginning of the universe. So, if it was the beginning (and I'm not saying it was) it wouldn't be possible without any pre-existing energy. Which would bring up the question of how that energy came to be. That question is where a lot of people believe that God came in. I personally can't find any actual good explanation for how there could be pre-existing energy, in that situation.