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News Feature: Lighting the way for dark matter

  1. Adam Mann, Science Writer

A nagging lack of evidence for weakly interacting massive particles has spurred physicists to start searching for a range of lightweight dark particles and even new dark forces.

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 dark matter, hinting that the mysterious substance is more complex than previously believed (2). Ideas about dark matter are evolving.

Using observations of gravitational lensing of galaxies, astronomers have mapped out dark matter in the Bullet Cluster, which is formed by two enormous colliding clusters of galaxies. Composite image courtesy of NASA/CXC/M. Weiss.

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 most-favored 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 …

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