For 20 years, an experiment in Italy known as DAMA has detected an oscillating signal that could be coming from dark matter—the fog of invisible particles that ostensibly fill the cosmos, sculpting everything else with their gravity.
One of the oldest and biggest experiments hunting for dark-matter particles, DAMA is alone in claiming to see them. It purports to pick up on rare interactions between the hypothesized particles and ordinary atoms. But if these dalliances between the visible and invisible worlds really do produce DAMA’s data, several other experiments would probably also have detected dark matter by now. They have not.
Late last month, Rita Bernabei of the University of Rome Tor Vergata, DAMA’s longtime leader, presented the results of an additional six years of measurements. She reported that DAMA’s signal looks as strong as ever. But researchers not involved with the experiment have since raised serious arguments against dark matter as the signal’s source.
DAMA searches for popular dark-matter candidates called WIMPs, or weakly interacting massive particles. The scientists monitor an array of sodium-iodide crystals kept deep under Gran Sasso Mountain in the Apennines, looking for flashes of radiation that could be caused by dark-matter particles colliding with atomic nuclei in the crystals. As the solar system hurtles through the galaxy, “it looks like a wind of WIMPs coming at you,” explains Katherine Freese, a physicist at the University of Michigan who in 1986 codeveloped the idea for such an experiment, “in the same way that when you’re driving it looks like the rain is coming into your windshield.”