I like the idea that super black holes create new universes as proposed by Smolin, somewhat to his embarrassment it seems. It couples nicely with some of the discussion in Brian Greene's book 'The Elegant Universe'.
As the story goes, the unified field exists inside the singularity of black holes. When that massless field becomes intense enough, it creates a new baby universe in a new big bang (for the baby universe) at the same time that it creates time and creates the space of the baby universe.
So it correct to say that the big bang started time and is still creating space for each universe. Yet we can as easily imaging a continuous existence of time back into the black hole.
I think the unified field is in 26 dimensions with two dimensions being time-like, as derived for 26-D string theory. So it is not clear how to think about time in the singularity of a black hole. But it seems reasonably to suggest that particular universes are finite in time and space; but that the sum of all universes are infinite in time and space.
So then what kind of nothingness does the big bang expand into. My guess is that it just pushes the existing space aside while not coupling to it.
Not sure about gravity. One claim of physics is the gravity came from the Higgs particle which is on the electro-weak energy scale. That is, it came rather late in the big bang. But that jives with recombination, the name of when matter annihilated all of anti-matter. There was so much matter before recombination, that if gravity existed then, the big bang would have immediately collapsed. [There clearly was no gravity during inflation. But that is very early in the big bang.]
The problem with such a late presence of gravity is that it cannot predict the rather fast formation of galaxies and clusters of them.
I think that Dark Energy in a sense is responsible for the rapid clumping of matter.
It's really Dark Matter that is responsible because in this hypothesis, dark matter is responsible for dark energy.
The hypothesis is that there is another charge C in the universe in addition to the electrical charge Q. It differs in that for C like charge attracts and unlike repels. And all matter possess this charge, especially dark matter, which after recombination, was most of the matter in the universe.
Furthermore, dark matter does not couple to photons. So it was free to clump much sooner in the big bang than ordinary matter was. But since gravity did not exist at this early time, it was like C charges attracting like C charges that began the clumping process. Hence, the universe ended up with different regions of plus C charge and minus C charge, separated by vast voids.
These regions of different C charge sign are still repelling each other and that in this hypothesis is what accounts for the accelerating expansion of the universe. That is, dark energy comes from dark matter.
So dark matter is both responsible for the early formation of galaxies and for the accelerating expansion of the universe.
Remember you heard it here first.
One last word. I used to worry that the C charge would immediately clump into black holes and thereby cease to exist. But if the C charge is associated with axions, and axions are the primary ingredient of dark matter, then each axion particle is so light in mass that its wave functions are at least global in extent and collapse of the resulting superfluid to a point is not possible.