Good points BB. Especially the one about what is outside the universe.
Because, now we can talk about three topic-related questions at once:
1) what is outside the universe?
2) where did the energy for the Big Bang come from?
3) where is the gravitational energy of black holes stored?
Obviously 2) and 3) are related, if the Big Bang came from some sort of "ultra-massive" black hole scenario, as proposed by previous posters.
In all three cases, the answer could be (without being facetious): SPACE...
Not empty space, because we are coming to realize more and more as time goes on that space is not empty. But rather, space under pressure.
By "space", I mean the vacuum medium, which permeates almost everything (except perhaps elementary particles) and is the seat of inertia, gravitation, electric and magnetic fields, etc.
In other words, suppose that "space" is a compressible, rotateable medium which can support gravitational pressure and magnetic and electric fields. That is why "space" has its own magnetic permeability (µo) and electric permittivity (εo), even when there are no other polarisable or magnetiseable materials present.
Thinking about what is outside the universe, consider the leading edge of the expanding wavefront from the former Big Bang (of 14 billion years ago), where we think everything is moving at the speed of light. In that region there could be a highly pressurised area of space, like a spherical shock-wave front. Beyond that, the pressure of the vacuum medium would fall to the ambient level of space, over some as yet unknown distance. Until the next universe is encountered...
As regards the energy that caused the Big Bang, if this was stored in the gravitational energy of an "ultra-massive black hole", then it would be the vacuum medium (space) itself which supplied the energy, via the monstrous gravitational pressures which it evidently can support in the vicinity of a black hole.
Okay, now if you did not choke on any of that, I take my hat off to you! Because that probably means you are a free thinker. You might also be aware that such a space or vacuum medium is perfectly in accord with Relativity, both Special and General (being Lorentz invariant, amongst other things), so it does not cause Michelsen or Morley any lost sleep in their respective graves.
Now, the big question for me is: how does matter cause gravitation, especially in a black hole?
Anyone like to propose a mechanism at the level of the vacuum medium?
Obviously to do that, one must first come up with a realistic model of the vacuum medium (space) itself, at a level down to, say, the Planck dimension...Is that asking too much?
Okay, now I am being facetious...
Perhaps elementary particles are some type of void or bubble in the space-time medium. It takes considerable energy to make a bubble in a pressurised medium. When enough massive particles collect in one place, the pressure of space itself pressing in on them, keeps them pressed together with an immense gravitational force, that may be nothing more than the pressure of the vacuum medium (space) itself. When the bubble gets to a certain "ultra-massive" size, or is catastrophically perturbed in some way, nothing can stop space imploding and producing a rather big bang (THE Big Bang?).
I will stop there.
Hopefully these ideas will seed yet more ideas and we can speculate freely in good company!