New theory : Dark energy -- the one that's attributed to cause the universe to expand as accelerated speed -- might be explained by
1. Bringing in a quantum level particle called "graviton" in picture. Which carries gravity. Unlike einestine's theory which says gravity isn't any energy, just distortion in space due to mass.
2. Giving gravitons a mass. Which limits their extend on how far gravity can propagate and thus explain accelerating speed of the universe.
All these are "theories" that are nice to know. No final solution to any of those problems yet. And recent research on neutron explosion says the universe might not be accelerating actually.