Open questionCosmology · Universe

Dark energy

Expansion is speeding up, and no one knows what drives it. Early hints suggest the effect may even change over time.

Open question · about 68% of the universe

What it is: The unknown cause of accelerating cosmic expansion
Share: Roughly 68 to 71% of the universe's energy
Simplest model: A cosmological constant
First evidence: Distant supernovae in 1998
Key surveys: DESI, Euclid, Rubin, Roman
Open clue: DESI hints the equation of state may shift with time

Expansion speeding up

NASA/STScI/G. Bacon

Artist concept of an accelerating universe driven by dark energy — An artist's concept of an accelerating universe driven by dark energy. Image: NASA/STScI/G. Bacon.

The discovery

In 1998 two teams measured distant type Ia supernovae and found them fainter than a steadily expanding universe should allow. The simplest reading was that expansion is not slowing under gravity but speeding up. The result was unexpected and was confirmed by later surveys.

The cause was given the name dark energy, a label for something we do not yet understand.

What it might be

The simplest option is a cosmological constant, a fixed energy tied to empty space itself. Alternatives include a dynamical field that changes with time, or a sign that general relativity needs a correction on the largest scales. Each option makes different predictions for how expansion evolves.

What the new surveys are finding

DESI maps millions of galaxies through baryon acoustic oscillations and has reported a hint that dark energy may be weakening rather than staying constant. If that holds, the cosmological constant is not the whole story. Euclid and later instruments will test the same question with independent data.

Key publications

Year	Paper	Venue	Link
1998	Riess et al., Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant	AJ 116, 1009	DOI
1999	Perlmutter et al., Measurements of Ω and Λ from 42 High-Redshift Supernovae	ApJ 517, 565	DOI
2024	DESI Collaboration, DESI 2024 VI: Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations	arXiv preprint	arXiv
2025	DESI Collaboration, DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints	arXiv preprint	arXiv

Selected papers for further reading. DOI links open the publisher's record; preprints are linked on arXiv.

Worth knowing

Dark energy is named for what we do not know about it.
It now dominates the universe's energy budget.
A constant version fits most data, but the latest maps suggest it may not be constant.