Dark energy is a mysterious form of energy that is thought to permeate all of space and is responsible for the accelerated expansion of the universe. It was first identified in the late 1990s when observations of distant supernovae showed that the universe was not only expanding but that its expansion was also accelerating.
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Dark energy is a smooth component with sufficiently negative pressure to drive accelerated cosmic expansion. The supernova discovery of accelerating expansion measures an expansion history through luminosity distance; interpreting that history as a component of the stress tensor additionally assumes general relativity. A cosmological constant is one possibility, whereas dynamical dark energy permits a changing equation-of-state parameter.