At first, I was going to write that there was no distinction, but that may not be quite right here.
For most ordinary effects, there's no distinction between saying "there's a new fundamental force" and saying "there's a new fundamental type of energy". That's probably easiest to see in a simple Newtonian picture. The fundamental forces are just given by spatial derivatives of the fundamental interaction energies.
The evidence for dark energy is that the expansion of the universe is accelerating. Within the current error bars, it's growing exponentially. That's exactly the behavior that's expected in General Relativity if every volume of space automatically contains a certain fixed energy density- the so-called dark energy. (Other causes can also have this effect, but let's stick with dark energy for now.) Now if you were to ask in what direction this dark energy was exerting a force, you couldn't point to one. Every place is surrounded uniformly and symmetrically by the dark energy. So it really isn't a force in anything like the normal sense of the word. One could stretch the sense of the word force, but then the distinction between it and the effects of energy again ceases to be important.
There are real questions, however, about whether the uniform energy density picture is correct, or whether one needs interactions with branes in other dimensions, or other models. (http://arxiv.org/abs/0901.0756
) Perhaps some model in which the overall boundary conditions are pulling outward on our universe could be described better by force language than by energy density language.
(published on 02/04/11)