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Dark matter and energy

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Home »<br>Astronomy and space » Dark matter and energy » Does the Bullet Cluster contain a lot less dark matter than we thought?

Dark matter and energy

Research update

Does the Bullet Cluster contain a lot less dark matter than we thought?

08 Jul 2026

Reading Time: 4 minutes

A different conclusion The Bullet Cluster, with hot gas in pink and perceived dark matter in blue. (Courtesy: X-ray: NASA/CXC/MMarkevitch et al.; Optical: NASA/STScI; Magellan/U Arizona/D Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U Arizona/D.Clowe et al.)">

A different conclusion The Bullet Cluster, with hot gas in pink and perceived dark matter in blue. (Courtesy: X-ray: NASA/CXC/MMarkevitch et al.; Optical: NASA/STScI; Magellan/U Arizona/D Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U Arizona/D.Clowe et al.)

The Bullet Cluster of galaxies might only have half as much dark matter than scientists thought, or perhaps even none at all, according to a new look at observations of this immense collision of galaxies from the James Webb Space Telescope (JWST).

“Our results indicate that the required amount of additional unseen mass – dark matter – is reduced,” astrophysicist Dong Zhang of the University of Bonn, who led the study, tells Physics World.

The Bullet Cluster is a central battleground for proponents of alternative models of gravity seeking to usurp dark-matter theory. The cluster itself is actually two clusters of hundreds of galaxies that have collided head-on. In 2006 the Hubble Space Telescope mapped how the Bullet Cluster’s mass was warping space and creating gravitational lensing, and from this it was possible to infer where the dark matter in the cluster was relative to the galaxies themselves and the hot, X-ray emitting gas. While the galaxies and hot gas remained in the middle, the dark matter from both colliding clusters had passed through unhindered and is now on opposite sides – exactly as dark-matter theory predicted.

In 2025 the JWST observed the Bullet Cluster and the initial analysis, led by Sangjun Cha of Yonsei University, found that while the distribution of mass in the cluster is more complex than expected, it still supports the dark-matter model.

“Those JWST observations of the Bullet Cluster have sparked a lot of interest,” says astrophysicist Richard Massey of Durham University, who was not involved in the new study.

Counting stellar remnants

Astrophysicists led by Zhang have now made their own analysis of the JWST data and arrived at a very different conclusion, as they report in Physical Review D.

Zhang’s team aimed to precisely measure the total mass of not just the stars, but also of dark stellar remnants – white dwarfs, neutron...