Since the 1980s physicists have deployed a string of increasingly advanced detectors in pursuit of something that ought to be ubiquitous but has proved devilishly hard to capture. Dark matter, the invisible stuff thought to make up a quarter of the universe, has yet to show in even the most sophisticated experiments.
Another blow befell the search last October, when the world's most sensitive detector of WIMPs (weakly interacting massive particles) came up empty. Dark matter may well be a WIMP, a ghostly particle that would interact with normal matter very infrequently, which is why at least 15 experiments around the globe are looking for the particles. But if those campaigns fail to hit particle pay dirt in the next few years, scientists may have to refocus the search and embrace alternative explanations for dark matter—some of which are less than appealing.
South Dakota's Large Underground Xenon (LUX) detector was the latest to take an unsuccessful swipe at WIMPs. Although WIMPs are elusive, occasionally one of the particles should collide with an atom inside LUX's 370-kilogram vat of liquid xenon, producing a detectable light signature. The researchers have seen nothing of the kind after three months, ruling out some possible characteristics for WIMPs, such as certain masses for the particles. By now more than half of the possible kinds of WIMPs that had been predicted have been eliminated, says LUX co-spokesperson Richard Gaitskell of Brown University.
The hopes of detecting dark matter are clouded by the possibility that it might not be a WIMP. Another candidate particle, the axion, would be much lighter than a WIMP and therefore more difficult to spot. “You don't hear about axions as much because it has proved somewhat harder to detect them,” says Stanford University physicist Peter Graham. Only one large-scale project is currently on the case.
An even thornier possibility is that dark matter only interacts with normal matter via gravity, meaning that snagging dark matter in a particle detector may be forever beyond our grasp. “That's the most pessimistic possibility, which we all hope it isn't,” Graham says.