Cosmology · Universe scale
Dark energy
An unknown form of energy that permeates all of space and drives the accelerating expansion of the universe. Makes up approximately 68% of the total energy content of the observable universe. Its nature is the deepest open question in cosmology.
Universe composition
Source: ESA Planck 2018 results
The accelerating universe
The discovery of dark energy was one of the most surprising results in the history of physics. The prevailing view in the 1990s was that the universe's expansion, set in motion by the Big Bang, was gradually slowing under the influence of gravity. Two teams set out to measure exactly how quickly it was decelerating using Type Ia supernovae as "standard candles" — objects with known intrinsic brightness. Instead of finding deceleration, both teams found the opposite: the expansion is speeding up.
The redshift of distant supernovae (measured via spectroscopy) tells us how fast they recede; their apparent brightness tells us their distance. If the expansion were constant, these two quantities follow a predictable relationship. The observed brightnesses were lower than expected — meaning the supernovae were farther away than a decelerating expansion would place them. Read more about how redshift is measured in our redshift dictionary entry.
Current measurements and open questions
Precision measurements of the Cosmic Microwave Background by the Planck satellite confirm that dark energy accounts for 68.3% of the total energy density of the universe. The CMB, combined with baryon acoustic oscillations (the "acoustic scale" imprinted on galaxy clustering) and Type Ia supernova data, gives a consistent picture of a flat universe dominated by dark energy.
Whether dark energy is truly constant — or slowly evolving — is the primary science goal of the ESA Euclid mission and the DESI survey. Our article on the Euclid dark universe map covers the first results from Euclid's wide-field survey.
Frequently asked questions
- In 1998, two independent teams studying Type Ia supernovae as distance markers found that distant supernovae appeared fainter than expected — meaning they were farther away than the then-standard model predicted. The only explanation consistent with the data was that the expansion of the universe is accelerating. Something was pushing galaxies apart faster and faster. That something was named dark energy. The discovery earned the 2011 Nobel Prize in Physics for Saul Perlmutter, Brian Schmidt, and Adam Riess. The Supernova Cosmology Project has the original data.
- The simplest model of dark energy is Einstein's cosmological constant (Λ, lambda) — a constant energy density filling space uniformly. Einstein originally introduced Λ in 1917 to balance gravity and produce a static universe; he later called it his "greatest blunder" when Hubble showed the universe was expanding. After 1998, Λ was reintroduced as the most economical explanation for the accelerating expansion. In the ΛCDM model (Lambda Cold Dark Matter), Λ has a constant equation-of-state parameter w = −1. Current data is consistent with w = −1 but not tightly constrained. ESA's Euclid mission is designed to measure w to 1% precision.
- No — they are entirely different phenomena. Dark matter is an invisible form of matter that exerts gravitational attraction, clustering in halos around galaxies. Dark energy is a form of energy with negative pressure that drives accelerating expansion — acting against gravity on the largest scales. In the standard cosmological model (ΛCDM): ordinary matter ≈ 5%, dark matter ≈ 27%, dark energy ≈ 68% of the universe's total energy content. They were discovered by different methods and have opposite effects on cosmic structure formation.
- This is one of the most important open questions in physics. If dark energy is the cosmological constant, it is by definition unchanging. But some models — called quintessence — predict it evolves over time, with the equation-of-state parameter w changing. In 2024, the DESI (Dark Energy Spectroscopic Instrument) survey released its first analysis of 6 million galaxy spectra and found preliminary hints that w may not equal −1, possibly varying with redshift. DESI's first-year results are a leading reference. If confirmed, this would fundamentally revise our model of the universe. See our article on the Euclid dark universe map for the latest observational constraints.
Browse all terms in the dictionary.
Open the Dictionary →