New Study of the Bullet Cluster Challenges the Existence of Dark Matter
Featured image: [Composite image of the Bullet Cluster showing X-ray gas in pink and gravitational lensing mass in blue; credit: NASA/CXC/STScI/ESO]
A new study led by the University of Bonn has reopened one of the most contentious debates in modern cosmology: whether dark matter is real. Using fresh observations from the James Webb Space Telescope, the team argues that the famous Bullet Cluster, long considered the strongest observational evidence for dark matter, can be explained without invoking the elusive substance at all.
The Bullet Cluster, a colossal collision of two galaxy clusters about 3.7 billion light-years away, has been a cornerstone of the dark matter hypothesis since its discovery. Observations showed that the hot X-ray gas from the collision lagged behind the clusters’ galaxies, while the gravitational lensing signal, a measure of total mass, tracked with the galaxies, not the gas. This separation was interpreted as proof that invisible, collisionless dark matter halos surrounded the galaxies and passed straight through the collision, while the gas was slowed by friction.
The new study, published in Physical Review D on July 1, challenges that interpretation.
What the study found
Led by Dong Zhang of the University of Bonn’s Helmholtz Institute of Radiation and Nuclear Physics, the team used new JWST photometry to re-estimate the total baryonic mass of the three brightest cluster galaxies at the Bullet Cluster’s core. Crucially, they applied the Integrated Galaxy-wide Initial Mass Function (IGIMF) theory, which predicts that early-type galaxies contain a higher proportion of massive stars that have since collapsed into invisible stellar remnants, neutron stars and black holes.
These remnants exert gravitational forces identical to what is often attributed to dark matter, but they are made of ordinary baryonic matter. By accounting for them, the team found that the observed gravitational lensing can be explained by Modified Newtonian Dynamics (MOND) alone, no dark matter required.
“The Bullet Cluster is actually particularly consistent with the MOND scenario,” Zhang said.
Implications for the standard model
Even within the standard dark matter paradigm, the study suggests that the inferred dark matter content in the Bullet Cluster would need to be reduced by roughly half. Co-author Pavel Kroupa of the University of Bonn and Charles University in Prague noted: “Even in the standard model, which assumes the existence of dark matter, its postulated quantity would have to be significantly reduced, by around half.”
The paper’s abstract states: “Independently of the validity of MOND, there is now an emerging perspective regarding dark matter models: to account for the observations, they seem to require less dark matter than previously anticipated.”
The debate continues
The Bullet Cluster has long been considered MOND’s biggest challenge. Proposed by Mordehai Milgrom roughly 40 years ago, MOND replaces dark matter by modifying gravity at low accelerations. Critics have argued that the offset between the Bullet Cluster’s lensing mass and its X-ray gas cannot be explained without collisionless dark matter.
The Bonn study argues that under MOND’s QUMOND formulation, phantom mass appears where accelerations are low, concentrated near the point-like galaxies rather than the diffuse gas, naturally producing the observed offset. Combined with the additional mass from invisible stellar remnants, the strong-lensing signal in the cluster’s core is fully accounted for.
The authors acknowledge significant caveats. The physical viability of their model depends on the spatial distribution and dynamic behavior of the stellar remnant population, which has not yet been established. The analysis also focuses only on the core regions where strong lensing operates; consistency at larger radii where weak lensing dominates requires further work.
The broader debate remains unresolved. MOND has historically struggled to explain cosmic-scale phenomena, including the cosmic microwave background and the large-scale structure of the universe. Nevertheless, this study demonstrates that the Bullet Cluster, long held up as proof of dark matter, is not as definitive as previously thought.
The paper, “Baryonic mass budgets in the central regions of the Bullet Cluster and their consistency with strong lensing in MOND,” is published in Physical Review D (DOI: 10.1103/6zrp-q7c4) and includes co-authors from the University of Portsmouth, Yonsei University, Charles University in Prague, and other institutions.