Dark energy is the name scientists give to whatever is causing the expansion of the Universe to accelerate. It is not known to be a conventional substance like atoms or ordinary matter. Instead, it may be a property of space itself or something associated with a new field of physics.
The Universe has been expanding since the early Universe. For a long time, astronomers expected gravity to gradually slow that expansion. Observations of distant Type Ia supernovae in the late 1990s revealed something surprising: cosmic expansion is accelerating.
Despite their similar names, dark energy and dark matter are very different. Dark matter behaves as an unseen source of gravity that helps form galaxies and large-scale structure. Dark energy is associated with the accelerating expansion of the Universe.
One leading explanation is Einstein's cosmological constant, usually written as Λ. In this model, empty space itself has a constant energy density. As the Universe expands, this component continues to affect the overall expansion.