Imagine you set up a ridiculously-powerful computer to simulate a universe - literally a particle-by-particle perfect simulation. (You might need this to be a fairly small universe, of course)
The simulation begins with everything in one tiny place and then it explodes outwards, cools down, matter starts to coagulate, etc. etc.
Within the simulation, there was no time before that universe's Big Bang. You could pause and even rewind the simulation and this could never be noticed from inside. The simulation only has 'knowledge' of what happens within the simulation.
Imagine your tiny universe evolves life, and it becomes intelligent. Can you imagine any way, any way at all, that that intelligent life could look at the simulated universe, and from it work out that it's a simulation? Can you think of a way they could find out what kind of computer it's running in? Can you imagine a way they could work out what the universe the computer exists in is like? Can you imagine any way, at all, in which the inhabitants of that universe could ever come to be aware of you yourself, unless you intervened and told them about yourself directly?
The difficulty that that simulated universe would have in working out how the computer works and what the rules of OUR universe are, are AT LEAST as great as the difficulties that we face in working out what, if anything, gave rise to our own universe. Questions like "What was before the Big Bang?" and "What's outside the Universe?" are at best almost impossibly difficult to answer, and at worst as meaningless as "Where's the end of a circle?"
That's why nobody's busy trying to find out. Now because nobody's interested, but because we don't even understand our own universe yet, so how the hell do we stand any chance of working out what's beyond it?