The Universe isn’t everything. It’s one out of the infinite number of universes existing at different phases of existence. Why is it commonly accepted that there is one universe?

This falls under the Earth-Centric category of mistake. Existence lies at the center of it all. Imagine a ball, filled with the ingredients of existence in a “raw” form, devoid of constraints of space-time. In this ball, existence is in this state of wait. Outside this ball, the laws of physics applies, because in this realm, time moves forward. This ball’s surface is where the big bangs occur, the potential existence hitting existence and time spawns an explosion that shoots away from the ball. Imagine our universe’s big bang, how we came into existence from something so incredibly tiny it goes off into negative infinity the closer you approach the exact beginning. This matter and this explosion came from this ball of potential existence and universes are born and sucked back in. Our universe is expanding, and will hit a point of maximum expansion, and then like a deflating balloon will rapidly get sucked back into the ball amongst the soup of potential existence.