'Dark Energy' is the term used to explain the force that is making the univeral expansion accelerate. According to recorded red shifts, the universe is expanding. Not only is it expanding, but the rate of expansion is accelerating. There must be some unexplained force causing this, but nobody can detect or understand this force. So physicists cal it 'dark energy,' because they can't find it.
The acceleration of the rate of expansion of the universe is clearly predicted by General Relativity. It's quite simple, really. The only reason modern physicists haven't figured it out is because they steadfastly refuse to accept the basic principle laid out in general relativity - that Space-time is a thing, it is shaped by gravity, and gravity is infinitely fast. It makes them think of aether and phlogiston, so they shy away from it.
Space-time is a thing. This is a natural result of general relativity and Einstein's field equations. They show that space-time has properties, that those properties change with the application of time and outside forces (mass causing gravity), and that it influences the behavior of other things (motion of particles). This is pretty much the definition of a thing.
Space-time can be easily thought of as a fluid. Density of matter causes increases in the gravity field, which cause changes in the density (curvature) of space-time. Time points in the direction of increased gravity. Space and time both contract as gravity increases. The both expand as gravity lessens. Thought of as a fluid, the inverse of the gravitic Lorenz factor is the temperature. The higher the gravity, the colder and more dense the fluid becomes. The lesser the gravity, the hotter and less dense the fluid becomes. (Incidentally, this density analogy also works perfectly to explain apparent time and space contraction with velocity - the local density rises as speed rises, like a bow wave.) The event horizon of a black hole can be thought of as the point at which fluid space-time changes phase and becomes a solid.
This clearly implies that in regions with lower gravity, time runs faster, and space is less dense. Consider the implications for the universe as a whole. Less dense areas flow faster through time. If the entire universe is expanding, this means that the voids between galaxies are expanding faster than the areas in and around the galaxies themselves. As the universe expands, the voids become larger, and the difference between the rate of time in the voids and in the galaxies inexorably increases, speeding the expansion of the voids. This appears to us as a slow acceleration of the rate of expansion over time.
No 'dark' force is required to explain why the rate of expansion of the universe is increasing. It simply flows from the principles of general relativity.