Dark matter is one of the most important and most mysterious questions in modern astronomy. Although dark matter cannot be seen or touched, it profoundly influences the formation and evolution of galaxies through gravity. For a long time, scientists have widely adopted the “cold dark matter” model to describe how cosmic structures form. However, as observational precision has improved, a number of small-scale phenomena have emerged that do not fully align with the predictions of this classical framework.
For example, in some dwarf galaxies, dark matter appears unusually “diffuse,” with a relatively low central density. In contrast, observations of strong gravitational lensing have revealed extremely dense dark matter substructures whose compactness far exceeds what traditional models would predict. These two types of phenomena have long coexisted, yet they are difficult to explain with a single physical mechanism.
Recently, a new study by physicists from Purple Mountain Observatory, CAS, has offered an intriguing answer: dark matter may not be a single component, but instead consist of particles with different masses.
The researchers propose a “two-component self-interacting dark matter” model. In this scenario, dark matter includes at least two types of particles—one heavier and one lighter—which interact not only through gravity but can also undergo direct collisions. It is this additional interaction that gives rise to a process known as “mass segregation.”
Put simply, over time, heavier dark matter particles tend to sink toward the centers of galaxies, while lighter particles preferentially diffuse outward. This process is analogous to what happens in star clusters, where massive stars migrate inward and low-mass stars move to larger radii.
Using high-resolution numerical simulations and detailed theoretical modeling, the research team found that this mass-segregation effect can naturally reproduce a wide range of observational results. In dwarf galaxies, it leads to dark-matter cores with low central densities, consistent with the latest observations of galaxy clustering. In more complex and denser environments, some dark matter halos gradually become more compact, forming high-density structures capable of producing strong gravitational lensing effects. More importantly, the model can significantly enhance the probability of small-scale gravitational lensing. Mass segregation increases the concentration of dark matter in key regions, making dark-matter substructures more effective at “magnifying” the light from background galaxies. This helps alleviate the long-standing problem that observations seem to show too many small-scale strong lensing events.
The researchers emphasize that their work suggests several seemingly contradicting small-scale cosmological anomalies may, in fact, point to the same conclusion: the internal properties of dark matter are richer and more complex than previously assumed.
As future astronomical surveys and gravitational lensing observations reach ever higher precision, scientists may be able to use these “cosmic magnifying glasses” to further test whether dark matter is truly composed of multiple components. Perhaps in the not-too-distant future, our understanding of dark matter will undergo a pivotal transformation.
This study represents the second work by the Purple Mountain Observatory research team on two-component self-interacting dark matter and was recently published in Science Bulletin. In an earlier study, the team systematically investigated the impact of mass segregation on the diversity of core density distributions in dwarf galaxies, with the results published in Physical Review D. The authors of the paper include: Daneng Yang, Yi-Zhong Fan, Siyuan Hou, and Yue-Lin Sming Tsai.
Purple Mountain Observatory of the Chinese Academy of Sciences is one of China’s leading institutions for dark matter research. It undertakes national research missions in the field of indirect dark matter detection based on the DAMPE (Wukong) satellite and has long been engaged in astrophysics and cosmology research, with significant impact in areas such as dark matter and galaxy evolution.
Science Bulletin