My pet theory is that gravity isn't real. It's just the effect of random chance. That's why it can't be unified with Quantum Mechanics: It isn't real. By random chance, all the particles in the universe are behaving as if they are attracted to each other in exactly the same way, as if there is some sort of inverse square Law of Gravity. But there isn't. Particle movement is normally random, and matter zooms outward, as dictated by thermodynamics, Quantum Mechanics, and Dark Energy. Previous universes similar to ours have lasted a few microseconds, or a few hours or days at most. But, just like someone can have a run of luck, and roll sixes over an over again, our universe has been rolling sixes for the last 14 billion years. It's incredibly improbable, but the universe is so old that it was inevitable that a run of luck such as this would happen at some point. But it is also inevitable that any run of luck ends. There is already evidence that our luck is running out: Distant galaxies are red-shifted in the spectrum, suggesting that gravity in the far reaches of the universe is starting to return to normal, and the universe "out there" is flying apart.