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Type Ia supernova 1994D (bright spot, lower left) on the outskirts of galaxy NGC 4526, imaged by Hubble
Type Ia supernova 1994D (bright spot, lower left) on the outskirts of galaxy NGC 4526, imaged by Hubble. Image: NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team · CC BY 4.0 · via Wikimedia Commons
Science & space · Across the cosmos · discovered 1998 – today

Dark Energy: Why Is the Universe Speeding Up?

Unresolved1998 CE – 2025 CEKitt Peak National Observatory, Arizona (DESI)

In 1998 two teams found that the expansion of the universe is accelerating. The force behind it makes up most of the cosmos, and we do not know what it is.

Astronomers expected gravity to be slowing the expansion of the universe. In 1998 two competing teams, measuring the brightness of distant exploding stars called Type Ia supernovae, found the opposite: the expansion is speeding up. Saul Perlmutter, Brian Schmidt and Adam Riess shared the 2011 Nobel Prize in Physics for the discovery. The unknown cause was named dark energy.

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How to read the labels

EstablishedSupported by strong, converging evidence.
ContestedEvidence exists but qualified scholars disagree.
UnresolvedThe question is open; evidence does not settle it.
LegendComes from myth, literature or tradition, not physical evidence.
SpeculativeA hypothesis with little direct evidence so far.

Hero image: Type Ia supernova 1994D (bright spot, lower left) on the outskirts of galaxy NGC 4526, imaged by Hubble. Image: NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team · CC BY 4.0 · via Wikimedia Commons. Evidence labels describe the written claims, not the image. Version 1 · updated 2026-10-22.