The source is known. Its the cosmological constant term in the field equations. Dark energy is just a popular name because it makes it sound mystical.
Granted there is likely more to be learned about the cosmological constant like whether it can be manipulated in places just like the stress-energy tensor can be just by moving matter around, or whether there is an actual field particle associated with it, but from the beginning it was a part of Einstein's field equations so really not only is accelerated expansion understood, but it was predicted by the results of the field equations for the inclusion of the constant long before evidence for it had been observed. As far as quantization of gravity goes, though there are theories that do that describing gravity with a real force exchange carrier, I have my doubts that any such gravity force exchange carrier or graviton really exists since even though the spacetime curvature is real, the force of gravitation is just an inertial force.
My speculation is that space and time are only valid as concepts on scales large enough that a graviton force exchange model isn't required to be valid. It would be the group behavior of the matter not force exchanges that determines the spacetime geometry on such a scale. On a small enough scale where gravity is so weak compared to the other forces that it essentially doesn't exist the locations momentum energy time for anything gets so fuzzy how could gravity which is spacetime geometry be meaningful anyway. In short it may be the case that no quantum gravity theory is ever valid. We can speculate all we want. I just think its important to be able to be clear on what we are portraying as fact Vs theory Vs speculation Vs religion.