However, if you can imagine the existence of more than one universe—a possible consequence of membrane theory or string theory—then it is possible that other universes, with other dimensions of space and time, could have existed before this universe.

It is a theory. Scientifically speaking, that makes it more powerful than fact. Facts are single pieces of information, while theories incorporate many, many facts into a conceptual model, which is then confirmed by a process of prediction, observation, and experiment. In science, individual facts can be weak and often turn out to be wrong, whereas theories are not easily disproved and are strongly supported by evidence.

The Big Bang theory has solid scientific evidence to support it, and its fundamental concepts have been scientifically proven to be correct. However, like all major theories in science, there are many details yet unproven and many questions still unanswered. These many important unknowns will continue to lead scientists to search for answers and make new discoveries, as they try to understand the cosmos.