It's possible that before the Big Bang, the universe was an infinite stretch of an ultrahot, dense material, persisting in a steady state until, for some reason, the Big Bang occured. This extra-dense universe may have been governed by quantum mechanics, the physics of the extremely small scale.
So, at the Big Bang, everything was denser and hotter than it is now, but there was no more an "outside" of it than there is today. ... No one knows exactly what was happening in the universe until 1 second after the Big Bang, when the universe cooled off enough for protons and neutrons to collide and stick together.
I hope this answers your question!
