The Big Bang Theory describes an event when existing space — much hotter, denser, and smaller at the time — suddenly and rapidly started stretching out. The primitive universe was a scalding goulash of tiny particles, light, and energy, but as it expanded, space cooled enough to allow important processes to occur, such as forming atoms and elements. The expansion continues today.

That's it. It doesn't suggest what the conditions were before expansion. It doesn't suppose what the universe is expanding into. It doesn't even explain what caused the expansion in the first place. And there are reasons why trying to imagine the event as an explosion can lead to some misinformed conclusions.

"No reputable scientist will claim that we understand in detail what happened at the exact moment when the universe began. We just don't," said Don Lincoln, senior scientist at Fermilab in Illinois, in a video. "In spite of the fact that we don't know everything about how the universe began, I'm constantly staggered by the fact that we know so much."