Scientists have substantial evidence that dark matter exists and is at least five times as abundant as ordinary matter. But its nature remains a mystery. The leading hypothesis is that at least some of its mass is composed of weakly interacting massive particles (WIMPs), which on Earth should occasionally bump into an atomic nucleus.
DAMA’s sodium iodide crystals should produce a flash of light if this happens in the detector. And although natural radioactivity also produces such flashes, DAMA’s claim to have detected WIMPs, first made in 1998, rests on the fact that the number of flashes produced per day has varied with the seasons.
This, they say, is exactly what is expected if the signal is produced by WIMPs that rain down on Earth as the Solar System moves through the Milky Way’s dark-matter halo. In this scenario, the number of particles crossing Earth should peak when the planet’s orbital motion lines up with that of the Sun, in early June, and should hit a low when its motion works against the Sun’s, in early December.
There is one big problem. “If it’s really dark matter, many other experiments should have seen it already,” says Thomas Schwetz-Mangold, a theoretical physicist at the Karlsruhe Institute of Technology in Germany — and none has. But at the same time, all attempts to find weaknesses in the DAMA experiment, such as environmental effects that the researchers had not taken into account, have failed. “The modulation signal is there,” says Kaixuan Ni at the University of California, San Diego, who works on a dark-matter experiment called XENON1T. “But how to interpret that signal — whether it’s from dark matter or something else — is not clear.”