We do not know what 'existed' 'before' the Big Bang, because if general relativity is our only guide, both time and space did not exist 'before' the Big Bang. There is no way of describing any cause-and-effect chains if time is no longer available to organize events. Some cosmologists imagine and theorize that 'before' the Big Bang there was a completely acausal quantum state occurring in some 'field' or 'mother space-time' and that the Big Bang represents for us the event in which our space-time first became an independent 'entity' with its own topology. Inflation then expanded this quantum bubble into our enormous space-time; a process that is still happening today. The concepts of 'container' and 'boundary' are useful to us only in thinking about situations in which space exists. This may not have been the case 'before' the Big Bang, so it MAY not be meaningful to frame questions in this way. This is a very common question posed by philosophers in the 11th - 13th centuries, but we know a lot more about PHYSICAL space now than we did then, and many of the debates are no longer meaningful. Especially when we frame the question either in terms of 3-dimensional space, or the full 4-dimensional space-time manifold which is the more correct way to do it.