What is dark energy?
Something makes the expansion of the universe speed up. It makes up most of the cosmos, and we don't know what it is.
The question
The universe is expanding: galaxies are moving apart, as Edwin Hubble showed in the 1920s. Gravity pulls everything together, so for most of the 20th century astronomers expected the expansion to be slowing down. The only question seemed to be how fast.
In 1998 two independent teams measured it using distant exploding stars, type Ia supernovae, which shine with a nearly known brightness and can serve as cosmic distance markers. Both found the opposite of what they expected: the expansion is speeding up. Something is pushing space apart. It was called dark energy, and according to current measurements it makes up about 70% of all the energy in the universe. What it is remains one of the biggest open questions in physics.
What we know
- The acceleration is well established. The 1998 discovery, by the teams of Saul Perlmutter and of Brian Schmidt and Adam Riess, received the 2011 Nobel Prize in Physics. It has since been confirmed by independent methods: the afterglow of the Big Bang, the distribution of galaxies, and the growth of cosmic structures.
- The simplest explanation is a cosmological constant: an energy belonging to empty space itself, the same everywhere and at all times. Einstein added such a term to his equations in 1917 and later abandoned it. With dark energy it returned, and the standard cosmological model, ΛCDM, is built on it.
- Dark energy began to dominate only fairly recently in cosmic history, a few billion years ago. Before that, the gravity of matter was stronger and expansion slowed down.
- Since 2024, the Dark Energy Spectroscopic Instrument (DESI), mapping the positions of millions of galaxies, has found hints that dark energy is not constant but weakening over time. Its 2025 results, combined with supernovae and the cosmic microwave background, prefer an evolving dark energy over a constant one, with a statistical significance of 2.8 to 4.2 sigma, depending on which supernova sample is used. That's suggestive but not yet the 5 sigma physicists require for a discovery.
Main hypotheses
Vacuum energy
Quantum physics says that empty space is not empty: it is filled with fluctuating fields that should carry energy. This is a natural candidate for a cosmological constant. The problem is the size: naive calculations give a value about 10^120 times larger than what is observed, perhaps the worst prediction in the history of physics.
A new field
Dark energy may be a field that changes slowly over time, sometimes called quintessence. Unlike a constant, it could weaken or strengthen, which is exactly what DESI's hints suggest.
Modified gravity
Perhaps there is no dark energy at all, and instead Einstein's general relativity needs to be modified on the largest scales. Many such theories have been proposed; observations of gravitational waves in 2017 already ruled out a large class of them.
Measurement effects
Some argue that part of the apparent acceleration or its evolution may come from how we analyse the data: for example, small systematic errors in supernova brightness, or assumptions of the standard model built into the analysis.
Where understanding ends
We can measure what dark energy does with growing precision. We have no idea what it is. The most natural theoretical explanation, the energy of the vacuum, gives a wrong answer by an absurd factor, and no one knows why the real value is so tiny yet not zero.
There is also the "coincidence" puzzle: why dark energy and matter happen to have comparable densities right now, when we are here to observe them, although they change at very different rates as the universe expands.
And the standard model is under pressure from more than one direction. Besides DESI's hints, there is the "Hubble tension": the current expansion rate measured nearby differs from the rate inferred from the early universe. Whether these are signs of new physics or of unrecognised errors is unknown.
What would settle it
- Confirmation or rejection of evolving dark energy at 5 sigma. DESI completed its planned five-year survey in April 2026, mapping more than 47 million galaxies and quasars; the first dark-energy results from the full data are expected in 2027. New surveys such as the Euclid space telescope and the Vera C. Rubin Observatory will add independent tests.
- Precise tests of gravity on cosmic scales that can distinguish dark energy from modified gravity.
- A theory that explains why the vacuum energy is so small, or a new field that predicts the measured behaviour before it is observed.
Sources
- Einstein, A. (1917). Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie. Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften, 142–152. (Scan of the original volume: Internet Archive.)
- Weinberg, S. (1989). The cosmological constant problem. Reviews of Modern Physics, 61(1), 1–23.
- Riess, A. G., et al. (1998). Observational evidence from supernovae for an accelerating universe and a cosmological constant. The Astronomical Journal, 116(3), 1009–1038.
- Perlmutter, S., et al. (1999). Measurements of Ω and Λ from 42 high-redshift supernovae. The Astrophysical Journal, 517(2), 565–586.
- Frieman, J. A., Turner, M. S., & Huterer, D. (2008). Dark energy and the accelerating universe. Annual Review of Astronomy and Astrophysics, 46, 385–432.
- Planck Collaboration (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy & Astrophysics, 641, A6.
- DESI Collaboration (2025). DESI DR2 results. II. Measurements of baryon acoustic oscillations and cosmological constraints. Physical Review D, 112(8), 083515. (Preprint: arXiv:2503.14738.)
- Fermilab & Berkeley Lab (2026). DESI completes planned 3D map of the universe and continues exploring. Press release, 17 April 2026.