The Economist - Dec. 8 - 2018

Still in the dark

A persistent claim to have detected dark matter looks wrong!

Negative results are rarely reported widely. But they can be important. And this week sees one such—the possible demolition of what had been thought to be a sign of dark matter.

Twenty years ago results from dama, a particle detector housed under a central-Italian mountain, suggested that this mysterious stuff, thought to make up 85% of all matter in the universe, had been detected, dama's operators saw an oscillating signal, peaking in June and at its lowest ebb in December, which was consistent with the Milky Way being embedded in a halo of dark matter. Because Earth's axis of rotation is tilted with respect to the disc of the Milky Way, the flow of dark matter through the planet should vary seasonally as it orbits the sun. Calculations suggest the dark-matter flux should reach a maximum in early summer and a minimum in winter, which is what dama found.

DAMA has continued to gather data— and more than two decades later and after numerous upgrades researchers there continue to see their cyclical signal. In the meantime, none of the other detectors set up to find dark matter has caught even a whiff of the stuff. But because these searches are conducted in ways different from dama, their results might somehow be explained away. So, in an effort to replicate precisely what dama does, yet more experiments have been set up, this time using the same detector material-crystals of sodium iodide, which should flash if dark-matter particles collide with them.

This week, one such experiment, COSINE-100, in the Yangyang Underground Laboratory, in South Korea, reports in Nature an analysis of data taken during its first two months of operation. Though not definitive (which would require a full year's data) this analysis suggests that the average signal over the collection period, minus noise such as that caused by the radioactive decay of the surrounding rock, leaves little room for a dark-matter signal akin to that at dama. The research therefore, continues — as does the mystery about dark matter’s true nature.