The Big Bang theory does not explain why the Big Bang actually happened. A well-established hypothesis is that the universe began in a “quantum foam”—a formless void where bubbles of matter, far smaller than atoms, were fluctuating in and out of existence on timescales far shorter than a trillionth of a trillionth of a trillionth of a second. In our universe today, such quantum fluctuations are thought to occur, but they happen so quickly that they never affect what happens in the cosmos. But if, 13.7 billion years ago, one particular fluctuation appeared but did not disappear, suddenly ballooning outward into a gigantic, explosive expansion, then it is possible that something like today’s universe could have been the eventual result.

In another, more recently proposed hypothesis, the universe is a four-dimensional spacetime that exists at the intersection of two five-dimensional structures called membranes. Picture two soap bubbles that come in contact with one another and stick together: The “skin” where the bubbles intersect is a two-dimensional result of the interaction between two three-dimensional structures. If the membrane hypothesis is correct, then the Big Bang event marked the moment the two membranes made contact. Neither of these models has any kind of experimental or observational confirmation yet.