What do I know, I just read Nature Physics and quantum theory textbooks, but I so don't have a physics degree (unlike my grandfather).
But here's an idea. And it does have the merit of extreme simplicity and not needing to invent shit we can't observe because they're too tiny or multidimensional.
We can't find gravitons or make them work, not because of anything mysterious about dimensions, but because there's only one graviton. The singularity like "point" that expanded and became our universe is just a graviton and it has a really really really low frequency called the spacetime continuum.
Dark energy that is sucking the universe somewhere, and all those galaxies appearing to accelerate and maybe disappear in one sector of the sky: that's the one antigraviton, which by definition is what the universe isn't (and it thus "outside" of it). Our graviton is just being attracted towards the antigraviton somehow, hence the acceleration.
When they touch, a universe destroying-creating energy will be released and there will be another graviton–antigraviton pair.
Black holes are like the fractal nature of radio waves observed in Bell Labs: there are little versions of the wave inside the wave, so your cell phone aerial needs to be crinkly, like maybe a Sierpinski carpet. The universe makes little versions of itself because that's the shape and activity of a graviton.