KiwiPhil, your question may very well represent a lot of people so could you please help me understand what you are actually asking about? I am starting to believe that we are all trying to answer a question that you never meant to ask! As I see it, there are two alternatives here:

1. You are one out of millions of otherwise well-informed people who have never understood the Big Bang theory (or relativity) well enough to realize what it claims. Your post #52 might imply that you think of the Big Bang as an explosion and that you believe in it only because you find it logical that we can calculate backwards to a point in time when that explosion occurred. Well, most of the answers that you have received on this forum try to convey that the Big Bang is not that at all and if this misconception of the Big Bang theory is the reason for your question (and you have my full respect because you are certainly not alone) then I believe you should not turn to this forum for the answer. You should read a few good books on the subject!

On the other hand you might be putting an entirely different question. In that case you have just not done a good job explaining that to us:

2. You fully understand and accept that the very foundation of the Big Bang theory is that the universe that we observe and the space that surrounds us and the time that we live in – the interwoven concept of space-time – all originates in a singularity with no dimensions at all. If that is your position, your question is rather whether the Big Bang that caused OUR universe could take place INSIDE another universe, another space-time. Then I suggest you read a little more till you realize that you have just asked: “What is south of the south pole?”. If that question still makes sense to you, I offer the answer “Nobody knows and nobody CAN know because our concepts of space and time are as confined to our own universe (read my post #46) as the concepts of north and south are confined to the surface of the Earth”.

So in the first case, you are putting a question that can be answered by the Big Bang theory if you take the time to study it. In the second case your question can never be answered. It would be most clarifying if you could tell which category is yours or if you can find yourself a third one.