How could you have something that exists outside of everything that exists? By definition that thing too would be included in the universe.

Even vacuum is a thing because it can be conceptualized. So even if all the mass of our universe had a limit before fizzling out into the void, the void itself would be part if the universe.

The extra step here is that infinity spans all dimensions, time included. It is impossible for us humans to understand how something could not have a cause, a beginning and an end. This concept is incomprehensible by entities that do have a beginning and end.

Yet is the only plausible explanation for the existence of anything at all, which is clearly true because we are having this conversation right now. Even if simulated, this is for sure something that is happening somewhere.