What Is Dark Energy, and How Do Scientists Actually Detect It?

Dark energy is the name astronomers give to whatever is making the expansion of the universe speed up. Nobody has detected it directly. It is inferred from how the universe expands and how its structure grows, and those measurements say it makes up about 68 percent of the total energy content of the cosmos. This page explains what is known, what the main explanations are, and how current surveys are testing them.

The Enigma of Dark Energy

According to the Planck satellite’s 2018 analysis of the cosmic microwave background, the universe today is about 68 percent dark energy, 27 percent dark matter and 5 percent ordinary matter, the atoms that make up stars, planets and people. Dark energy behaves like an energy belonging to space itself. Unlike matter, it does not dilute as the universe expands, and because it has negative pressure its gravitational effect is repulsive: it pushes the expansion to accelerate.

The discovery came in 1998. Two independent teams, the Supernova Cosmology Project led by Saul Perlmutter and the High-z Supernova Search Team led by Brian Schmidt with Adam Riess, measured the distances of distant exploding stars of a type whose true brightness can be calibrated. Both expected to find that gravity had slowed the expansion over billions of years. Instead, the distant supernovae were fainter than expected, meaning the expansion is speeding up. The three leaders shared the 2011 Nobel Prize in Physics, and the cosmologist Michael Turner coined the term “dark energy” for the unknown cause.

The supernova result has since been confirmed by independent methods: the pattern of hot and cold spots in the cosmic microwave background, the characteristic spacing of galaxies left by sound waves in the early universe (baryon acoustic oscillations), and the slower than expected growth of galaxy clusters.

Theories Illuminating the Shadows

Physicists have several candidate explanations. The key observable separating them is the equation of state parameter w, the ratio of dark energy’s pressure to its energy density.

1. Cosmological constant. Einstein added this term to general relativity in 1917 for a different reason, to hold the universe static, and dropped it in 1931 after the expansion was discovered. Today it describes a constant energy density of empty space, with w exactly equal to −1. It is the simplest explanation and the one built into the standard cosmological model, but quantum field theory estimates of the vacuum energy come out vastly larger than the observed value, a mismatch nobody has resolved.
2. Quintessence: a dynamic scalar field that fills space and whose energy density can change over time, so w can differ from −1 and evolve.
3. Modified gravity: theories that change general relativity on cosmological scales, so that the acceleration comes from gravity behaving differently rather than from a new component.
4. Phantom energy: a hypothetical form with w below −1, whose density would grow as the universe expands. If it existed, the acceleration would increase without limit and eventually tear apart galaxies, stars and atoms in a “Big Rip”.

Current data are consistent with a cosmological constant, with some recent hints of evolution described below. None of the alternatives has been confirmed.

Distinguishing Dark Energy from Dark Matter

Dark energy and dark matter share the word “dark” and little else. Dark matter behaves like ordinary matter in one respect: it clumps under gravity. It is detected through its gravitational pull on stars, galaxies and light, and it forms the invisible scaffolding on which galaxies and clusters are built. As far as anyone knows it does not emit or absorb light, and it has not yet been identified as a particle.

Dark energy does not clump. It appears to be spread almost evenly through space, and its effect is the opposite of dark matter’s: instead of pulling structures together it drives the expansion that pulls galaxy clusters apart from each other. Its effect became dominant only in the last several billion years, as matter thinned out while dark energy did not.

Both are inferred from observations rather than detected in a laboratory. Particle physicists search for dark matter in underground detectors and at colliders such as the LHC; no comparable direct experiment exists yet for dark energy.

Illuminating the Shadows: Missions and Observations

A new generation of surveys is measuring the expansion history and the growth of structure precisely enough to test whether dark energy changes over time.

The Dark Energy Spectroscopic Instrument (DESI), on a telescope in Arizona, is mapping the positions of tens of millions of galaxies and quasars. Its results released in March 2025, based on nearly 15 million objects and combined with supernova and cosmic microwave background data, prefer a dark energy that weakens over time over a pure cosmological constant, at a statistical significance between 2.8 and 4.2 sigma depending on the data combined. That is a strong hint, short of the five sigma level physicists usually require for a discovery, and DESI’s own data alone remain consistent with the standard model.

ESA’s Euclid space telescope, launched on 1 July 2023, is imaging billions of galaxies to measure how dark matter and dark energy have shaped the universe over the last ten billion years. It published its first quick data release in March 2025. On the ground, the Vera C. Rubin Observatory in Chile began its ten-year Legacy Survey of Space and Time in June 2026, photographing the whole southern sky every few nights and finding large numbers of the supernovae that first revealed the acceleration.

Together these surveys should show within the next few years whether the DESI hint grows into evidence that dark energy evolves, which would rule out a simple cosmological constant, or fades away.