Even if you follow your logic and our universe was expanding into another volume, you can ask the same question of that — where did it come from, ad infinum.

But even worse, that would all be speculation because anything “outside” our universe is inaccessible to observation anyway. By definition our universe is all matter, energy, space and time contained therein. There is no “outside”.

Your question has been asked many times with regards to the balloon analogy. “But where is the balloon a inflating into?” In the analogy, that’s higher dimensions. But that’s the rub — it’s just an analogy and not literal. Space “is” and expands in a measurable way. Always. Using an analogy (like the balloon) and concluding “must”, well, that’s a fallacy. The analogy helps describe some aspects of what we observe, but it’s not reality.

As for the Big Bang, all energy was contained in a singularity that was a state beyond which our physics can describe. Time, space, forces, and energy were all unified. That’s why there was no “before” the Big Bang because just like there was no space, there was no time. All of our dimensions, space and time, were created with the Big Bang.

One last note, quantum field fluctuations and uncertainty are natural part of or laws of physics. It’s real and tested. “What triggered” the Big Bang may simply be random quantum wave fluctuations in whatever state the universe was in. No special action was necessary. Our current understanding of physics simply isn’t advanced enough to describe that state. Not without quantum descriptions for gravity and maybe time too.