Neutrinos produced by Fermilab are directed in a beam by a set of two special lenses and are then detected by two detectors - a 'near' one in Fermilab, and a 'far' one in the Soudan lab, 735 km away.

Credit: Fermilab

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ON AN EARLY summer morning in northern Minnesota, a crew of about a dozen waits by the top of mine shaft No. 8. Donning hard hats, the engineers and physicists pile into a creaky, double-decker elevator cage.

It is pitch black for most of the three-minute descent. Ears pop, the cage floor vibrates and a giant motor dating from 1925 thunders overhead. When the cage door slides open, the team is 713 m below the surface.

Directly ahead lies a maze of tunnels - an abandoned mine where labourers once extracted iron ore of uncommon purity.

But the scientific crew takes a U-turn into a huge and unexpectedly spacious two room cavern known as the Soudan Underground Laboratory.

The workers have journeyed deep into the Earth to plumb the darkest depths of the cosmos, hunting for the missing material believed to account for around 83% of the universe's mass.

That material, known as dark matter, must exist, astronomers say, because the cosmic allotment of ordinary, visible matter doesn't provide enough gravitational glue to hold galaxies together.

Although the missing material shouldn't be any more prevalent in the underworld than above ground, dark matter hunters have good reason to frequent Soudan and other subterranean lairs.

Because dark matter particles would interact so weakly, experiments designed to detect the dark stuff could easily be overwhelmed by the cacophony of other particles. So scientists use Earth's crust to filter out cosmic rays - charged particles from space that bombard Earth's atmosphere.

Physicists have been directly searching for dark matter for more than two decades. But until recently, only one experiment, beneath a mountain in central Italy, had consistently reported evidence of the invisible particles.

Now two more experiments have found similar hints. When taken together, the findings suggest that the most popular models for dark matter may not be correct - the particles pegged have a lower mass than many physicists had proposed.

"Any discovery of dark matter would be a major revolution," says theorist Neal Weiner of New York University. "But if these results are right, I think it's even more exciting than that."

If the low-mass measurements are confirmed, a second revolution is in the making: in addition to dark matter, a new force may be needed to explain the workings of the universe. Favourite theories may require revision or may even have to be discarded.

But not so fast, some scientists say. Other recent work questions whether researchers have actually spotted low-mass dark matter particles. And with so much at stake, including the likelihood of a Nobel Prize for whoever discovers dark matter first, rival teams have resorted to name-calling.

One team has twice publicly ridiculed the results of a second, while the team whose analysis has come under fire has likened the attacks to the Spanish Inquisition. It's an exciting but confusing time, Weiner says.

Physicists have long had a leading model for dark matter. They believe that it consists of a proposed particle left over from the Big Bang called the WIMP, for weakly interacting massive particle.

WIMPs sense only gravity and the weak force, the interaction that governs radioactive decay. Particle physicists like WIMPs because they fit neatly into a theory known as supersymmetry, which unifies the two basic types of elementary particles - force carriers and matter particles.

The supersymmetry theory requires that every force carrier has a heavier matter particle for a partner, and every matter particle has a heavier force-carrying partner, doubling the number of particles in nature. The lightest supersymmetric partner would be stable, making it an ideal candidate for the WIMP that physicists propose.

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