Galaxy clusters, the largest structures held together by gravity in the universe, may be far heavier than scientists once believed.
New research suggests that much of their “missing” mass could be explained not by exotic dark matter alone, but by vast numbers of stellar remnants—neutron stars and black holes left behind when massive stars die.
The study was led by researchers at the University of Bonn under the direction of astrophysicist Pavel Kroupa, who works at the Helmholtz Institute for Radiation and Nuclear Physics.
Their findings were published in the journal Physical Review D.
Galaxy clusters contain hundreds or even thousands of galaxies, along with hot gas and other material. For decades, astronomers have struggled to fully account for the mass needed to explain how these enormous systems stay bound together.
Previous estimates suggested that large amounts of unseen matter must be present, leading to the widespread idea that dark matter dominates galaxy clusters.
In the new study, the researchers took a closer look at how stars form and evolve within galaxies that make up clusters.
They used a framework known as the Integrated Galaxy-wide Initial Mass Function, or IGIMF, a theory developed in Bonn that describes how stars of different masses form across entire galaxies.
This approach allows scientists to more realistically estimate how many massive stars form and, crucially, how many neutron stars and stellar black holes they eventually leave behind.
By applying the IGIMF theory and combining it with a large body of observational data, the team recalculated the stellar populations of many galaxies in different clusters.
The data included measurements from gravitational lensing, where massive objects bend light from distant sources, as well as detailed observations of individual galaxies within clusters.
The results showed that galaxy clusters are, on average, about twice as massive as earlier estimates suggested. Much of this additional mass comes from neutron stars and black holes, which are extremely dense but difficult to detect directly.
These stellar remnants also help explain the observed abundance of heavy chemical elements in clusters, which are produced in large quantities by massive stars before they explode.
Interestingly, the newly calculated cluster masses align well with predictions from Modified Newtonian Dynamics, often called MOND, an alternative theory of gravity proposed to explain cosmic motions without invoking large amounts of dark matter. In contrast, when using standard Newtonian gravity combined with dark matter, the required amount of dark matter turns out to be significantly lower than previously assumed.
Although the research is fundamental in nature, the authors say it offers new insight into the deep relationship between matter and gravity. By improving our understanding of how mass is distributed in the universe, the findings may eventually influence future theories of space-time and, potentially, lead to new technological advances.
For now, the study suggests that the universe’s missing mass problem may not be as mysterious as once thought—much of it could be hiding in plain sight, locked inside the remnants of long-dead stars.
