Dark Energy
A mysterious force or property of space causing the universe's expansion to accelerate. Makes up about 68% of total mass-energy.
Dark energy is the term for whatever is causing the observed acceleration of the universe's expansion. Discovered in 1998 from observations of distant Type Ia supernovae, it constitutes about 68% of the universe's mass-energy — yet its nature remains unknown.
The simplest model is the cosmological constant (Λ), an energy density inherent to space itself, predicted by Einstein's general relativity equations. Alternative models include quintessence (a dynamic field varying over time and space) and modifications to gravity at very large scales.
If dark energy is truly constant, the universe will continue accelerating forever, eventually leading to a "Big Freeze." If it changes over time, other fates become possible.