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I'm surprised nobody mentioned the idea of inflation yet, which I think often is considered before the Big Bang (also a well established and accepted theory)
Basically the basic setup is that you have some energy stored in some quantum field. This field is unstable, so it causes the universe to expand exponentially. At some point this field decays, the universe heats up, reaches some maximum temperature, and then Big Bang begins.
Now if you get to the nitty gritty, it's plausible that the Universe has been forever inflating, where pockets of the universe decay, forming bubbles of universe and one of which may be the one we're living in. This is the idea of https://en.wikipedia.org/wiki/Eternal%5C_inflation[1][2] which I think is quite a compelling hypothesis for our Universe.
1: https://en.wikipedia.org/wiki/Eternal%5C_inflation
2: https://en.wikipedia.org/wiki/Eternal_inflation
There's nothing here!