This article is my current favorite hypothesis for what caused the Big Bang.
In my head, the collision of the ‘branes is a bit like when you accidentally hit the clamps of active jumper-cables together. There’s a mad sparking, which shoots out, fizzles, and disappears. Only in this case, it’s very slow-mo from our point of view. Everything we know of – stars, planets, us, etc., is the result of that sparking collision, and someday the universe will reach essential emptiness again as the spark fizzles out and dies. Is that a satisfactory zero energy state?
If so, then… not anytime soon. But then, it’s not in any rush.