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Scientists are fond of saying negative results are just as important as positive results, but after several decades of not finding something, researchers can be forgiven for feeling impatient. Back in the 1990s, experiments began trying to detect the particles that make up dark matter, the ubiquitous yet untouchable invisible material that apparently fills the cosmos. Since then, physicists have found more and more evidence that dark matter is real but not a single sign of the stuff itself. A new version of the long-running XENON experiment that started up late last year aims to finally break that pattern.

One of physicists' best guesses about the identity of dark matter has long been that it is made of particles called WIMPs—weakly interacting, massive particles. These elementary bits of matter could be anywhere between the mass of the proton and 1,000 times the mass of the proton, and they would interact with regular atoms only through gravity and the weak nuclear force, which governs radioactivity. But over the years, as experiment after experiment failed to find anything, some of the enthusiasm has faded. “You do start to scratch your head and think maybe that was the wrong horse to bet on,” says Rafael Lang, a physicist at Purdue University, who has been working on the XENON experiment at the Gran Sasso National Laboratory in Italy for more than a decade. For now, though, Lang says he is keeping his money on WIMPs, pointing out that the experiments have falsified many of the theories predicting what WIMPs might look like but certainly not all. “If you believed in WIMPs 10 years ago, only half of those WIMPs have been ruled out,” he says. “The other half are still alive.”

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