Dark energy is the name given to the force that is believed to be making the universe larger. Distant galaxies appear to be moving away from us at high speed: the idea is that the universe is getting bigger and has been since the Big Bang. Measurements are now good enough to allow astronomers to tell that these galaxies seem to be accelerating away from us. The universe is expanding at an ever-increasing rate.
This faster and faster expansion is not understood. There are many ideas for what might be causing it, but at the moment we do not know enough to say whether any of them are right. It can be said that it is as if there was something there in empty space providing a repulsive force that makes the universe expand. This has been named dark energy.
This reference, which is way beyond what we can discuss on Simple wiki, sets out the problem for theoretical physics.