News Feature: Lighting the way for dark matter.

In 2015 a team of nuclear physicists in Hungary reported an anomalous bump in the signal from radioactive decays of unstable beryllium-8, corresponding to a putative new particle with 34 times the mass of an electron (1). It was largely overlooked at the time, but a year later US theorists suggested that this might point to a new force felt by darkmatter, hinting that themysterious substance ismore complex than previously believed (2). Ideas about dark matter are evolving. Since the late 1990s most researchers have posited that dark matter is probably made of weakly interacting massive particles (WIMPs): ghostly hypothetical objects that would pass through normal matter like light through a glass window. Entities with perfectly WIMP-like properties happen to appear in supersymmetry, a popular theory extending the Standard Model, the current framework describing the interactions of all known particles and forces. But experiments searching for these WIMPs have so far turned up empty, and the Large Hadron Collider has failed to reveal any signs of supersymmetry. Although supersymmetric WIMPs remain the mostfavored candidate for dark matter, their nonappearance has led some scientists to begin doubting their existence and explore numerous new models. A few physicists are turning to another kind of particle, an ultralight entity known as the axion. Others suggest that there might be many distinct dark matter particles, each with unique properties, which could combine into dark atoms and dark molecules and emit dark photons. “This generalization from a particle to a