The universe is expanding because the black hole that is our universe continues to accrete matter and energy from the 'outside' parent universe, increasing in mass, and increasing its Schwarzschild radius. The cosmic background radiation (CMB) is Hawking radiation from the massive hollow shell that makes up the event horizon. The event horizon appears to be a hollow shell because it was hugely separated from our location in the early expansion phase by nearly infinite length contraction in the black hole gravity well. The interior spacetime curvature has since relaxed to almost nothing and a flat space remains due to expansion. The CMB started out as a blackbody spectrum with a peak wavelength in the gamma ray range, but has been redshifted due the the rapid expansion billions of years ago. We must be very far from the event horizon because if we were closer then the Hawking radiation that is still being produced would not be as redshifted as it is. All the matter in the universe is also Hawking radiation, produced from virtual particles at the event horizon. The huge curvature of spacetime outside the event horizon versus the nearly flat curvature of spacetime inside the event horizon has somehow resulted in preferential selection of particles versus antiparticles to be evaporated to the interior space of our black hole universe.

It is not necessary for dark energy to exist. Although all the matter in the universe would start out as fundamental particles (dark matter) before agglomerating into galaxies and forming stars. There is no such thing as gravitons, there are only relativistic effects. That means the event horizons that form around black holes are purely mathematical phenomena and 100 percent efficient. No losses are possible. The process of renewal is a perpetual motion machine.

We have no idea how much more mass our universe will pull in from outside. But the CMB forms an absolute frame of reference with respect to velocity. And the absolute magnitude of its reshifted spectrum might indicate how far away we are from the event horizon. If the expansion of the universe is accelerating, it is because our black hole universe just happens to be accreting a particularly large clump of mass now - it could slow down later.

I really like this theory. It's incredibly consistent and simple.