Dark matter is believed to make up a large fraction of the matter in the universe, yet its true nature remains unknown. Most past experiments have focused on heavier dark matter candidates, while much lighter dark matter, with masses closer to the mass of a neutrino, has been difficult to detect directly because its scattering signals are extremely weak.

An international team of researchers has found that torsion-balance experiments —precision instruments originally built to test the equivalence principle—can double as detectors for very light dark matter. The study, published in Physical Review Letters, provides the strongest direct detection limits to date on interactions between dark matter and nucleons in this mass range from about 0.01 to 1 eV.

The team of researchers, including The University of Tokyo Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) Professor Shigeki Matsumoto and Kavli IPMU Todai Postdoctoral Research Fellow and JSPS Fellow Jie Sheng, focused on one key physical effect: when dark matter is sufficiently light, its number density in a galaxy becomes very high, and its scattering cross section with macroscopic objects can also be greatly enhanced by coherent effects.

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Berlin, Asher, Foster, Joshua W., Hooper, Dan, Krnjaic, Gordan.

A study led by UC Riverside physicist Hai-Bo Yu suggests that a new type of dark matter could explain three astrophysical puzzles across vastly different environments. Published in Physical Review Letters, the study proposes that dense clumps of self-interacting dark matter (SIDM)—each about a million times the mass of the sun—can account for unusual gravitational effects observed in gravitational lenses, stellar streams, and satellite galaxies.

Dark matter, which makes up about 85% of the universe’s matter, cannot be seen directly. The standard model assumes it is “cold” and collisionless, meaning that particles pass through one another without interacting. This model struggles, however, to explain certain high-density structures observed in the universe.

Yu’s work instead focuses on SIDM, in which dark matter particles collide and exchange energy. These interactions can trigger “gravothermal collapse,” forming extremely dense, compact cores.