I've been thinking about this lately. We still don't know what "Dark Energy" really is, most probably think its some extremely advanced uber-quantum stuff. But what if dark energy is something much more simple?
What if that "dark energy" is basically cosmic radiation that pushes against the universe (or space-time?) as its a giant solar sail? And since more and more matter turns into radiation, this effect accelerates the expansion of the universe.
Or what if "Dark Energy" is just the kinetic energy from the big bang? But that would not explain the acceleration.
What do you think?