Zitat Sabine Hossenfelder@skdh A group of astrophysicists has reanalyzed the masses of galaxy clusters and they say that we might not need dark matter after all. 1:47 PM · Apr 26, 2026
Zitat The Dark Matter mystery just took a weird turn. A group of astrophysicists reanalyzed data of galaxy clusters and they say that the mass we've been told is missing doesn't seem to be missing after all. Well, that's quite an unexpected answer to an almost 100-year old problem. I've had a look at the paper.
Dark Matter was first discovered in galaxy clusters in the early 1930s, so the new study is a neat return to the beginnings of this story. Galaxy clusters are the largest gravitationally-bound structures in the universe. A typical cluster can contain hundreds of even thousands of galaxies, all orbiting within a huge cloud of hot gas. The average velocity of the galaxies in the cluster depends on the total mass of the cluster. This is basically because more galaxies lumped together means that the total gravitational pull is larger, and that means the galaxies can sped up more. Observations shows that galaxies and clusters just move much faster than they should if we add up all the matter. (Graph: Normal matter 4%, dark matter 24%, dark energy 74.4%)
This is one of of the key reasons why astrophysicists introduced Dark Matter - an invisible substance that outweighs normal matter by about a factor of 5 on average. Galaxy clusters play a special role in this story because observations we have of them are particularly difficult to explain by alternative ideas. The most prominent alternative is called Modified Newtonian Dynamics (MOND for short). Instead of adding Dark Matter, Mond modifies the law of gravity when accelerations become extremely small, which happens in galaxies and also in galaxy clusters. And MOND works surprisingly well for many galaxies. This is its great appeal.
But when it comes to galaxy clusters, MOND doesn't properly work. It just gives the wrong predictions. They aren't hugely off, but by about a factor of 2, either one has to adjust the way that gravity is modified, or one has to add Dark Matter on top of it. Or so we thought.
The authors of the new paper analyzed observational data from 47 galaxy clusters, including gravitational lensing measurements, X-ray data and optical data. Then they calculated how many stars of different mases should form in those galaxy clusters. The most massive stars do not live very long. They quickly explode as supernovae and leave behind neutron stars and black holes. These remnants are difficult to detect, but they still carry mass, so what the authors do is basically calculate how many of these dark remnants should exist in galaxy clusters, and from this they conclude that clusters must contain almost twice as much mass in stars and stellar remnants as early estimates suggested.
In other words: there IS a lot of invisible matter in clusters, but it's not exotic dark matter. It's ordinary matter in a very unglamorous form: Dead stars. And the relevance is that now with the revised mass estimates the MOND predictions are much better compatible with observations. This is because in MOND the amount of missing matter smaller. You don't need 5 times the amount of mass to explain galaxy cluster dynamics. In MOND you just need about twice as much. And that's what they found. They say: "We've underestimated the normal mater in galaxy clusters. It's not a perfect fit, but it dramatically alleviates the tension between MOND and cluster dynamics." In contrast, the authors argue, that the standard Dark Matter interpretation does not fit as neatly once the stellar population corrections are included. As the sum it up in the press release:
"The newly determined masses are in good agreement with the predictions of Milgrom's theory of gravity, MOND. Newton's theory with Dark Matter, on the other hand, does not provide correct results as the amount of Dark Matter required for this is only about half as much as previously assumed."
So, does MOND suddenly work after all? What are we to make of this? For one thing, it must be said that these calculations are based on theoretical assumptions about star formation. These are indirectly based on observations, of course, but not directly, so I expect that some astrophysicists will disagree. There is also the question of whether this explanation will work for all clusters, or only for the one studied in this analysis.
And MOND has some other issues. We have gravitational lensing, wide binaries, or the supposed galaxies that don't contain Dark Matter. Though the relevance of these observations depends strongly on whom you ask. Generally, if my time in astrophysics has taught me one thing, it's much, much more difficult to properly interpret and analyze data in astrophysics than it is in particle physics. This is why the field seems to progress so slowly. It's really got a data aggregation problem, or: on the bullshit meter gives it a 7 out of 10. Calculating the expected number of stellar remnants and checking it against observations is certainly a valid idea. But the claim that this resolves the cluster problem for MOND seems ... optimistic, at this stage. If nothing else, this paper is a reminder that before inventing a whole new substance to explain our observations, it's worth checking whether we counted the corpses correctly.
Pressemitteilung:
Zitat Stellar remnants solve the mystery of missing mass in galaxy clusters +++ RESEARCH TICKER UNIVERSITY OF BONN: Astrophysics +++
Under the leadership of the University of Bonn, a research team led by Prof. Dr. Pavel Kroupa from the Helmholtz Institute for Radiation and Nuclear Physics has discovered that galaxy clusters are about twice as heavy as previously assumed. The additional mass comes mainly from neutron stars and stellar black holes and also explains the observed quantities of heavy elements.
WHAT IS IT ABOUT? Galaxy clusters are the largest gravitationally bound structures in the universe, but their mass and composition have not yet been fully researched. A new study led by Prof. Dr. Kroupa investigated how stellar populations and matter are distributed in the clusters. Using the Integrated Galaxy-wide Initial Mass Function (IGIMF) theory developed in Bonn, realistic star populations were calculated and the total masses of the clusters were determined.
WHAT IS THE MOST IMPORTANT FINDING? The study shows that galaxy clusters are about twice as heavy as previously assumed. The newly determined masses are in good agreement with the predictions of Milgrom's theory of gravity (MOND). Newton's theory with dark matter, on the other hand, does not provide correct results, as the amount of dark matter required for this is only about half as much as previously assumed.
WHAT WAS THE CHALLENGE? For the study, the researchers had to compile a large amount of observational data on numerous galaxy clusters, including data from gravitational lens measurements and detailed information about the individual galaxies. On this basis, the star populations of each galaxy had to be recalculated in order to finally determine the total mass of the clusters.
IS THERE AN APPLICATION? No, this is purely basic research. However, the results provide a new understanding of the connection between space-time and matter, which could potentially lead to new technologies in the future.
WHO WAS INVOLVED? In addition to the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn, the Astronomical Institute of Charles University in Prague and the Institute for Advanced Studies in Basic Sciences (IASBS) at the University of Zanjan, Iran, were also involved in the study.
WHAT IS THE SOURCE? Dong Zhang, Akram Hasani Zonoozi, and Pavel Kroupa: Revisiting the missing mass problem in MOND for nearby galaxy clusters, Physical Review D (PRD), DOI: https://doi.org/10.1103/mp3f-q5dc, advance copy via arXiv: https://arxiv.org/abs/2602.06082
"Revisiting the missing mass problem in MOND for nearby galaxy clusters" - Dong Zhang (Bonn), Akram Hasani Zonoozi (Bonn, Zanjan), Pavel Kroupa (Bonn, Prague) (Phys. Rev. D 113, 043027)
Zitat Abstract
In the framework of Milgromian dynamics (MOND), galaxy clusters have been thought to have about a factor of two less baryonic mass than gravitational mass. One hypothesized source of this missing mass is undetected baryons. Extensive observations and studies indicate that the baryon content of galaxy clusters is primarily composed of the intracluster medium (ICM). In this work we reevaluate the overall stellar mass in galaxy clusters taking into account recent work on the galaxy-wide stellar initial mass function of stars (gwIMF) needed to synthesise the metals observed in galaxies. Given their supersolar metallicities and short formation timescales, massive elliptical galaxies are inferred to have formed with highly top-heavy gwIMFs, which in turn leave behind a substantial mass in stellar remnants. The dependency of the gwIMF on the properties and evolution of a galaxy is well encapsulated by the integrated galaxy-wide initial mass function (IGIMF) theory, developed independently of MOND. We utilize observational data at redshifts z<0.1 from the Wide-field Nearby Galaxy-cluster Survey (WINGS) and the Two Micron All Sky Survey (2MASS). Masses of galaxies and intracluster light (ICL) are calculated for 46 galaxy clusters using the IGIMF theory. The resulting masses in stars and in remnants are combined with previously derived ICM masses to estimate the total baryonic masses of the clusters. These baryonic masses are then compared to the MOND dynamical masses of the clusters, which are derived from hydrostatic equilibrium of the ICM based on earlier studies. As a complement, we include a comparison with several weak/strong lensing masses in the MOND framework. Our results show that the stellar masses of galaxies and the ICL increase substantially when applying the galaxy-wide mass-to-light ratios derived from the IGIMF theory. This leads to a significant rise in the estimated baryonic masses of galaxy clusters. In the sample of 46 galaxy clusters, the baryonic component on average accounts for 52+4−3% of the MOND dynamical mass when considering only the ICM contribution. The baryonic mass in stars, remnants and the ICM accounts for at least 88+5+2−4−1% of the MOND dynamical mass. The contribution by stellar remnants that arises from nucleosynthesis constraints thus significantly alleviates the missing mass problem in MOND. Finally, we briefly discuss the compatibility of the IGIMF framework with the radial acceleration relation (RAR), and studies of MOND weak/strong lensing and related issues. A more comprehensive investigation will require future work that combines the IGIMF with self-consistent, spatially resolved formation, evolution, and resulting mass distribution models of galaxies.
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