A modified theory of gravity that incorporates quantum effects can explain a trio of puzzling astronomical observations: large scale structures, the horzon problem and the Pioneer anomaly.
The theory, called scalar-tensor-vector gravity (STVG), adds quantum effects to Einstein's theory of general relativity such that quantum fluctuations can affect the force felt between interacting objects. It does depend on a hypothetical particle called a graviton – which mediates gravity – appears in large numbers out of the vacuum of space in regions crowded with massive objects such as stars.
Any theory of gravity must explain the development of large-scale structures in the universe, and most importantly, the afterglow of the big bang called the cosmic microwave background (CMB) radiation (see "#2 Horizon Problem" of "13 things that do not make sense").
I am going to keep my eye on STVG, as it may offer a viable alternative to Big Bang Inflation theories of the variations in the speed of light and other universal "constants".