COUPP's "detector" is a glass jar filled with a liter or so of a fire-extinguishing liquid (iodotrifluoromethane). When a WIMP hits a nucleus of one of those atoms, it triggers an evaporation of a small amount of that liquid, producing a tiny bubble.
It's initially too tiny to see, but it grows, and that growth can be recorded with digital cameras.
"The bubbles in the fluid are slow enough that high-speed cameras will capture the changes through continuous still shots. We're making the world's most boring movie," Peter Cooper, the Fermilab physicist heading up CITRE, told Symmetry Breaking.
Once the bubbles reach about one millimeter in size, the COUPP scientists can study the images in earnest, looking for telltale statistical variations between photographs. Ideally, this enables them to distinguish whether a bubble resulted from background radiation, or from a dark matter particle.
COUPP currently operates at SNOLAB in Canada, another underground physics facility. The group has succeeded in placing some fundamental limits on certain properties for WIMPs.