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The Bullet Cluster (1E 0657-56) in a 2025 NASA Webb and Chandra composite: hot X-ray gas (pink) lags behind the mass mapped by gravitational lensing (blue)
The Bullet Cluster (1E 0657-56) in a 2025 NASA Webb and Chandra composite: hot X-ray gas (pink) lags behind the mass mapped by gravitational lensing (blue). Image: NASA, ESA, CSA, STScI, CXC; science: James Jee (Yonsei University/UC Davis), Sangjun Cha (Yonsei University), Kyle Finner (Caltech/IPAC) · Public domain · via Wikimedia Commons
Science & space · Across the cosmos · studied 1933 – today

Dark Matter: The Missing Mass of the Universe

Unresolved1933 CE – 2025 CESanford Underground Research Facility, South Dakota (LZ detector)

Galaxies spin too fast and clusters bend light too strongly for the matter we can see. Something invisible outweighs it — and no one has caught it.

In 1933 the astronomer Fritz Zwicky found that galaxies in the Coma Cluster move too fast to be held together by their visible matter. In the 1970s Vera Rubin and Kent Ford measured the rotation of spiral galaxies and found their outer stars orbiting as fast as the inner ones, as if wrapped in a halo of unseen mass. Astronomers call that missing mass dark matter.

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Connected dossiers

Dark Energy: Why Is the Universe Speeding Up?Same open question · Together, dark matter and dark energy make up about 95% of the universe.Webb's First Deep FieldComparable case · Webb refined the Bullet Cluster mass map in 2025.CERN and the Large Hadron ColliderSame open question · Particle colliders also search for dark matter candidates.

See Dark Matter in the discovery universe →

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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: The Bullet Cluster (1E 0657-56) in a 2025 NASA Webb and Chandra composite: hot X-ray gas (pink) lags behind the mass mapped by gravitational lensing (blue). Image: NASA, ESA, CSA, STScI, CXC; science: James Jee (Yonsei University/UC Davis), Sangjun Cha (Yonsei University), Kyle Finner (Caltech/IPAC) · Public domain · via Wikimedia Commons. Evidence labels describe the written claims, not the image. Version 1 · updated 2026-10-22.