Artistic impression of collisions between dark matter particles. One collision per particle every 10 billion years explains the distribution of dark matter in ultra-faint dwarf galaxies.
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Ultra-faint dwarf galaxies, among the tiniest and faintest galaxies known, may hold the key to understanding one of the Universe’s biggest mysteries: the true nature of dark matter. A new study reveals that even a single collision between dark matter particles every 10 billion years — roughly the age of the Universe — is enough to explain the dark matter cores observed in these small systems.
These galaxies, which contain only a few thousand stars, are dominated by dark matter and have relatively simple evolutionary histories. That makes them ideal cosmic laboratories for testing theories about dark matter physics. The study finds that extremely rare self-interactions between dark matter particles can naturally produce the central structures, or “cores,” that are observed — structures that standard, collisionless dark matter models cannot easily reproduce.
“We know that the current model of dark matter is only an approximation,” said the study’s author, Jorge Sanchez Almeida. “All particles, including dark matter particles, eventually interact through forces beyond gravity. Our study shows that even extremely rare interactions can leave observable marks on the smallest galaxies.”
By analyzing the sizes of stellar and dark matter cores in these galaxies, the researcher derived the range of self-interaction cross-sections — a measure of how likely dark matter particles are to collide. The study shows that both low-interaction (core-forming) and high-interaction (core-collapsing) dark matter halos could reproduce the observed structures, with cross-section values ranging from 0.3 to 200 cm2 per gram. These values are consistent with those found in other galaxies but now extend the constraints to objects where there is no alternative explanation.
The team also developed a simple model linking a galaxy’s stellar mass to its core radius — two properties that can be measured observationally. The model successfully reproduces the core sizes and predicts that the core radius increases with stellar mass, a trend also seen in larger dwarf galaxies. This relationship provides a powerful tool to connect visible structures in galaxies to the invisible properties of dark matter.
If the high-interaction scenario is correct, dark matter in ultra-faint dwarfs becomes thermalized over regions spanning roughly one kiloparsec, far beyond the visible extent of their stars. Such large thermalization scales could influence how dark matter substructures form and evolve inside more massive galaxies, potentially affecting phenomena like gravitational lensing and the distribution of satellite galaxies.
“Even extremely rare collisions between dark matter particles leave a lasting mark on the smallest galaxies,” the author noted. “Our results show that tiny galaxies can provide a direct window into the physics of dark matter.”
These findings position ultra-faint dwarf galaxies as unique and powerful natural laboratories for studying dark matter interactions at very low velocities — a regime inaccessible to both particle accelerators on Earth and observations of massive galaxy clusters.
Future cosmological simulations that include large self-interaction cross-sections will be essential to explore how these ultra-rare collisions shape galaxies across cosmic time. Such work could help refine our understanding of dark matter and, ultimately, of the fundamental forces that govern the Universe.
Article: Sánchez Almeida, Jorge. “Constraints on dark matter models from the stellar cores observed in ultra-faint dwarf galaxies: Self-interacting dark matter”, Astronomy and Astrophysics, 2025. DOI: https://doi.org/10.1051/0004-6361/202557040
Contact:
Jorge Sánchez Almeida, jorge.sanchez.almeida [at] iac.es (jorge[dot]sanchez[dot]almeida[at]iac[dot]es)
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