A whopping 96% of the essence of our universe lies in the dark sector, where essence refers to everything that controls evolution and large-scale properties of the cosmos.
Dark energy, on the other hand, is not a form of matter at all (nor is it literally "dark" -- that is just a poetic way of saying that it is mysterious). It is the source of the outward acceleration of the cosmos, and, based on reasonable assumptions about our current understanding of elementary particle physics, may arise from the vacuum, which has quantum properties that provide energy to the cosmos. Alternatively, dark energy may be a feature of gravity that produces cosmic repulsion on a large scale.
In working out their self-consistent theory of dark matter, the scientists find that a new, fifth force of nature is implied. Although this might seem an extravagant solution, they argue that it is in fact a relatively straightforward solution, fits into a very reasonable (if novel) picture of particle physics, and fortuitously appears to solve a number of independent problems. The new ideas, if confirmed, would revolutionize our understanding of the universe. This paper, which outlines the basic ingredients of the new model, lays out a concrete first step that subsequent research can now either contradict ... or confirm.
Then... it follows that there is some sort of potentially limitless energy that smart-enough creatures could eventually tap into... perhaps. Maybe only sci-fi now but it illustrates how little is known about the Universe.
Who could possibly argue that there are no more really great discoveries that could fundamentally change the world?