Generally these kinds of questions are not addressed by physical theories (how the continuum was generated, for example). Even the braneworld models that Drakkith mentions generally just assume that the branes are their from the beginning. One important thing to point out, however, is that modern cosmology does not necessarily place much significance in the big bang as being *the* beginning of all there is. All we know, observationally, is that our observable universe arose from a hot, dense state. Our observable universe should not be identified with *the* universe (there's probably more out there waiting to be observed). With the advent of the inflationary universe paradigm (in which the observable universe underwent an ultra-rapid exponential expansion early on), it is perhaps more natural to envision the observable universe as just one small "island" patch amid a larger universe, parts of which are still undergoing inflation (this is called "eternal inflation"). With this view, the big bang was an event local to our observable universe.