...where did all the energy/matter in the universe/multiverse come from?
Ex nihilo. If nothingness is unstable, there is a non-zero (albeit terrifyingly miniscule) probability for a patch of negative pressure energy to come into being; once that happens, the process of inflation gets underway and extends to spatial infinity and towards temporal (future, at least) eternity, giving birth to infinitely many bubble universes along the way.
Very interesting, I never heard of such concept. Of course, since i don't pretent to understand cosmology, that should come as no surprise.