Dark matter is really hard to find

First results from LUX dark matter detector rule out some candidates

It sounds like a big, expensive experiment that may well turn up nothing.  Still, the challenges of finding dark matter are huge:

Though dark matter has not yet been detected directly, scientists are fairly certain that it exists. Without its gravitational influence, galaxies and galaxy clusters would simply fly apart into the vastness of space. But because dark matter does not emit or reflect light, and its interactions with other forms of matter are vanishingly rare, it is exceedingly difficult to spot.

"To give some idea of how small the probability of having a dark matter particle interact, imagine firing one dark matter particle into a block of lead," Gaitskell said. "In order to get a 50-50 chance of the particle interacting with the lead, the block would need to stretch for about 200 light years—this is 50 times farther than the nearest star to the Earth aside from the sun. So it's an incredibly rare interaction."

Capturing those interactions requires an incredibly sensitive detector. The key part of the LUX is a third of a ton of supercooled xenon in a tank festooned with light sensors, each capable of detecting a single photon at a time. When a particle interacts with the xenon, it creates a tiny flash of light and an ion charge, both of which are picked up by the sensors.

To minimize extraneous interactions not due to dark matter, the detector must be shielded from background radiation and cosmic rays. For that reason, the LUX is located 4,850 feet underground, submerged in 71,600 gallons of pure de-ionized water.

But even in that fortress of solitude, occasional background interactions still happen. It's the job of LUX physicists to separate the signal from the noise.