Dark matter can't be seen and it's never been physically measured. But it's thought to consist of about four-fifths of all matter in the cosmos.
The mysterious substance is comprised of neutrinos, tiny and numerous particles that pass through us every second. Detecting these mysterious, almost theoretical objects is taking scientists to some unusual places.
Inside a hollowed-out mountain in France, along an Italian Alpine highway tunnel and in mines in the United States, Japan, and Canada, nuclear physicists use the natural shielding effect of solid rock to enable dark matter and neutrino experiments.
"We have to go way underground to get away from cosmic radiation," says Nigel Smith the director of the Canadian facility, the Sudbury Neutrino Observatory, or SNOlab. "Basically the cosmic rays hit the top of the atmosphere, they create a shower of particles through the atmosphere. These particles get soaked up as they go through the rock. That means what does get down here is probably what we're looking for."
Two kilometres down a nickel mine owned by Vale, a Brazilian company, SNOlab is a 5,000-square-metre warren of corridors, cavities and rooms gouged from the rock. About 300 people work there. To reach it, the researchers - who come from 55 universities and science institutions - have to ride a clanking elevator with real miners, walk through a rocky tunnel for 15 minutes, and then scrub themselves clean of the thick dust of the mine shaft.
Only then can they turn their attention to the various experiments in SNOlab. Four of them are aimed at finding dark matter, and two are monitoring neutrinos. There are plans for others.
Clarence Virtue of Sudbury's Laurentian University runs a project called HALO - basically an early-warning system for astronomers who do research on the vast celestial events known as supernovas.
"When a star implodes at the beginning of a supernova, it expels neutrinos even before the light from the blast is released. So when we detect neutrinos in HALO, we can tell that a supernova is about to occur and train our instruments on it," says Virtue. "We have anywhere from 30 minutes to 10 hours for a head start."
Scientists believe that such massive stellar destruction helps new celestial bodies form and keeps the universe in equilibrium.
"All of the heavy elements that formed the stars and the planets came from a supernova," says Virtue. "They were dispersed in space and available for the formation of planets. To understand how these things are born, we need to understand supernovas in detail."
HALO is essentially a stack of glass tubes full of radioactive helium, surrounded by solid lead bars. Neutrinos hit the lead and cause a reaction in the helium, a flash of energy.
The same principle of indirect observation lies behind the search for dark matter.
"If you look at wind moving a flag, you don't see the wind, you see the flag moving," explains Smith. "It's the interaction of the wind on something that you can actually see. And it's the same for our detectors here."
Some of the dark matter detectors use inert gases like argon and neon; others are based on rare elements like germanium. Something passing through the relevant material will cause a flash of light.
Actually a particle of dark matter moving through those substances is probably years away, says Kim Palladino, a post-doctoral researcher at the Massachusetts Institute of Technology.
"We're like the guy who drops his keys while walking home in the dark," Palladino asserts. "We're only looking for them in the light from the street lamp. Actually getting out there and searching in the dark will be much, much harder. And if we only look where it's easiest, it'll take much longer to find anything."
Nor is it entirely clear what the researchers are looking for.
Dark matter is thought to be made up of relatively large particles known, probably with a glimmer of science humour, as WIMPs. That stands for Weakly Interactive Massive Particles.
For an as-yet-unknown reason, they're invisible to humans but the patterns of gravity and the motion of stars in our galaxy suggest that there's far more out there than meets the eye.
In fact, without dark matter, our galaxy, solar system and planet couldn't have coalesced into the shapes and positions they take today. And that means that we wouldn't be here either.
"If we understand dark matter," says physicist Nigel Smith, "we will understand how the galaxies form, how the universe itself evolved from the Big Bang and moving forward through time. Understanding dark matter will allow us not only to understand the particle we think dark matter is made from, but even basic, fundamental things like why the galaxy is here today".
Physicists like Smith become quite emphatic, even a little emotional, when explaining why they search for something as elusive and difficult to explain as a WIMP.
"When we finally nail this, it'll be a big day. It'll be inspirational to scientists and engineers now and into the future, and inspiring people is why pure science like this is funded," Smith says.
As for a practical application for dark matter, scientists don't know what it would be. First find it, they say, then figure out what to do with it. "It may be that [dark matter] will have some use that we can't see now. And people will look back and say, 'Thankfully we understood the importance of dark matter and we looked for it back then,''' says Smith.
For MIT's Palladino, the pleasures of the search are of a more immediate type. Quite simply, she loves her job.
"It's such a cool thing to come down here in this lab every day and look for dark matter," she says. "It's so much better than sitting in an office by a window, typing on a computer. There, the good science is only in your mind. Here it's all around you. It's the coolest thing in the world."