Dark energy
The unknown component, about 68% of the universe, whose negative pressure drives the accelerating cosmic expansion.
Definition
Dark energy is the name given to whatever is causing the expansion of the universe to accelerate. It was discovered in 1998, when two independent teams measuring distances to Type Ia supernovae found that distant supernovae are fainter — and therefore farther away — than any decelerating universe would put them. Instead of slowing under the gravity of its matter, cosmic expansion is speeding up, which requires a component with negative pressure spread smoothly through space.
The simplest description of dark energy is Einstein's cosmological constant Λ, a constant energy density of the vacuum with equation of state w = p/(ρc²) = −1. Fit to the data, it corresponds to Λ ≈ 1.1 × 10⁻⁵² m⁻² and an energy density of only about 6 × 10⁻¹⁰ J/m³ — a few hydrogen atoms' worth of rest energy per cubic metre. Unlike matter and radiation, this density does not dilute as space expands, so dark energy comes to dominate the energy budget only at late times.
Dark energy is the deepest open problem in fundamental physics. Quantum field theory predicts a vacuum energy larger than the observed value by some 120 orders of magnitude — 'the worst prediction in the history of physics.' Whether dark energy is a true constant (w = −1 exactly) or a slowly evolving field (quintessence, w ≠ −1) is being tested by surveys such as DES, DESI, Euclid, and the Rubin Observatory's LSST. As of the mid-2020s the data are consistent with a constant, with intriguing but not yet decisive hints of evolution.