Assuming that the big bang initiated the existence of the universe as we know it, we ask what existed prior to the big bang? Was there space with no matter? Was there purely space? Or was there purely time, but no space or matter? Einstein's relativity answers this question: nothing existed before the big bang, not matter, not space, not even time. Space and time are different, but inseparable manifestations of the same physical realtity. Nothing caused the big bang, for it was not even a physical event itself that could be caused by another physical event. Nothing could have caused it because absolutely nothing preceded it within our spacetime.

No one can explain why spacetime itself came into existence. Here a theological question arises. God might be hypothesized as the creator of all spacetime. He or She would have existed outside of both space and time. The nature of the big bang therefore is a topic of theoretical supposition. Understanding the properties of the universe during the first fraction of a second after the big bang demands a quantum theory of gravity that takes into consideration enormous magnitudes of temperature, pressure, and density. Because we have no such unified theory that relates energy, matter and gravity, we can only speculate about 1) whether the big bang was indeed the beginning of spacetime, 2) if it was the beginning, whether spacetime will expand forever, or 3) whether the big bang was only a part of a previous sequence of expansions and contractions.