Phenomena · Space Codex
Dark matter and dark energy
Ordinary matter makes up only about 5% of the universe.
The 95% we cannot see
At a glance
Key points
The rest is unseen dark matter, about 26%, and dark energy, about 69%, which is accelerating cosmic expansion.
- Ordinary matter is about 5% of the universe
- Dark energy drives accelerating expansion
- DESI hints dark energy may change over time
Evidence
Galaxies spin too fast for their visible mass, galaxy clusters bend light more than their stars can explain, and the CMB's pattern requires extra unseen matter. Distant supernovae revealed that expansion is speeding up.
From Zwicky to Rubin
In 1933 Fritz Zwicky found that galaxies in the Coma cluster move far too fast to be held together by their visible stars. In the 1970s Vera Rubin and Kent Ford showed that stars in the outer parts of spiral galaxies orbit as fast as those further in, implying vast haloes of unseen mass. In 2006 lensing maps of the Bullet Cluster showed most of the mass of two colliding clusters separated from their hot gas, which is hard to explain without dark matter.
The hunt for particles
Detectors deep underground wait for a dark-matter particle to nudge an atomic nucleus, and particle colliders look for signs of new particles; none has found a convincing signal. Candidates range from heavy weakly interacting particles to extremely light axions. Some physicists instead propose modifying gravity, though such theories struggle to match the CMB and colliding clusters.
Mapping dark energy
DESI finished its planned five-year survey in April 2026 and is continuing into 2028; its first dark-energy results from the full data set are expected in 2027. ESA's Euclid, launched in 2023, and NASA's Roman Space Telescope, launched on 30 August 2026, will measure how dark energy has shaped the growth of cosmic structure over billions of years.
What we still don't know
What dark matter is made of, and whether dark energy is a constant or something that changes over time, as recent DESI results hint.
Missions
- 2009–2013
Planck
ESACompletedMapped the cosmic microwave background at the highest precision to date.
Related entries
- Cosmic microwave backgroundThe oldest light in the universe
- The cosmic webFilaments, voids and Laniakea
Sources
- Rubin & Ford 1970, ApJ 159:379 (opens in a new tab)
- ESA: Euclid (opens in a new tab)
- Planck Collaboration 2018, A&A 641:A6 (arXiv) (opens in a new tab)
- DESI Collaboration 2025 (arXiv) (opens in a new tab)
- Zwicky 1933, republished in English: Gen. Rel. Grav. 41:207 (opens in a new tab)
- Chandra: NASA finds direct proof of dark matter (opens in a new tab)
- IFAE: DESI completes planned 3D map of the universe (Apr 2026) (opens in a new tab)
- NASA Science: Roman Space Telescope launches (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.