Dark energy is a hypothetical form of energy that is proposed by physicists to explain why the universe is not just expanding but is doing so at an accelerating rate. The dominating form of energy in the cosmos, dark energy is driving the accelerating expansion of the universe, but its nature remains a complete mystery. In short terms: - Dark energy measures universe's expansion to 11 billion years ago. - Repulsive force that is the dominant component (69.4 %) of the universe. The remaining portion of universe consists of ordinary matter and dark matter. - In contrast to both forms of matter, is relatively uniform in time and space and is gravitationally repulsive, not attractive, within the volume it occupies. - The nature of dark energy is still not well understood.