A black hole, according to Juan Maldacena, can be described by the 2-D event horizon surrounding it, every bit of information, spin, charge, etc, is mapped onto the horizon with one bit corresponding to one planck area of the event horizon. What this means is as a black hole consumes matter or energy, that information is added to the area of the event horizon, and because the event horizon is 2-d, and because the interior of a black hole is volume, as the size of a black hole grows, the density falls.
If our Earth were made into a black hole, it would be about the size of a golf ball, and that event horizon would contain all the information about every particle that comprised the earth. If you were to grab a salt sized granule of that black hole to measure its density, it would be trillions of tons per cubic millimeter. However, the black hole a the center of the Milky Way has a radius about the size of the orbit of jupiter, and the millions of solar masses give an average density of that black hole, not trillions of tons per cubic mm, but it has the same density as water.
It just so happens that if you took every scrap of matter and energy in the universe and dropped it into a black hole, the size of the black hole would end up being, the same size as the universe at a density of one hydrogen atom per cubic meter.
Here is where it gets really weird. By some estimates, the size of our universe appears oof our universe is about 15 billion light years away. There is a short fall of the apparent size compared to the actual size, determined by the cosmological event horizon. Now, let's go back to a small black hole, say the Earth crunched down to the size of a golf ball. If you add one electron, the size of that black hole would increase as one bit of information was added to a new planck area, one for charge, the other for spin. The black hole grew in size by two planck areas. So, what does that mean for the inside of the black hole? It must stretch inside to accomodate the new size, a formerly small black hole grew in size by two planck areas.
So, if our universe is really the inside of a black hole, and since the natural Schwarzschild radius is 15 billion light years, but the size appears to be 13.4 billion light years, the inside of that black hole, our universe, stretches to fill the space. This could explain dark energy. Our universe is expanding to its natural Schwarzschild radius pulling the galaxies apart, but once the univers crosses the cosmological event horizon, passes the Schwarzschild radius, the dark energy would decrease and expansion would slow, until eventually billions of years from now, it started to contract. Since space appears to be a superfluid, or have the properties of superfluid, the universe would oscillate and appear like a Big Crunch was happening. Scientists of that day would think our universe was headed towards a Big Crunch because these things happen so slowly, they probably wouldn't be aware of an earlier time when the universe was expanding. They would probably, and rightfully surmise the universe was contracting due to gravity. But the surprise is the universe could be expanding because of gravity.
Gravity may have its anti-gravity force, the outward pressure from a black hole event horizon that is growing in size. It would be impossible to use that anti-gravity as it would only be apparent from inside the black hole. I read a book about the theory, and it seems to make a really good thought experiment case, although the black hole we live in wouldn't be a normal black hole, it would be a 4-d black hole, because space is not 3-d, we live in a 3-sphere, where every point within the 3 sphere is equidistant to every other point within the sphere. A 2-sphere is defined by an object where every point of its surface is equidistant to the center. A 3 sphere is very hard to imagine, but seeing how every point in our universe appears to the be the center, that the cosmological event horizon is 15 billion light years away no matter how far you travel. Even if you traveled 15 billion light years, the event horizon would always be the same distance away. A 3-sphere. Maybe our universe is a 4-D star that collapsed in a universe very unlike our own, a universe that has stellar objects that are 3-spheres, and maybe that 4-d star collapsed and created our universe. Our parent universe may be an exotic universe very unlike our own. Maybe there as many of those parent universe as there are universe like ours, like quantum mechanics would suggest, many universes with exotic properties not seen in our universe.
I have not heard an adequate argument against our universe existing on the inside a black hole, except some, like Sean Carrol saying "we don't live inside a black hole. And who would want to live inside a black hole anyway?" That is not an argument. Its an insult wrapped in an opinion. The universe doesn't care of Sean Carrol doesn't like the truth, or anyone. It just is what it is. If Carrol has the math or even a cogent thought experiment, I would be more likely to agree, but as it stands, it seems that this could be a real possibility. New studies on dark energy seem to indicate that its not just a case of vaccuum energy. The strange fluctuations of dark energy over time don't comport with with a vaccuum energy explanation. Maybe its something entirely different.