Results from the first physics-run of China's PandaX-II dark matter experiment are in and it doesn't look good. According to a paper published last week in the Physical Review Letters, no dark matter candidates have been observed, at least that can be isolated from background noise. The results further refine the limits on dark matter-normal matter interactions possible for theorized weakly-interacting massive particles—WIMPs, far and away the most popular dark matter theory—which is a good thing, generally, but still nothing like an actual detection event. The results align well with those from two similar direct detection experiments: LUX and XENON100.
The PandaX experiment lives at the China Jinping Underground Laboratory, which is advertised as the deepest underground laboratory in the world (at 2,400 meters). The depth is key because the more layers of dirt and rock that separate the detector from the atmosphere and space beyond, the more sheltered it will be from background noise—non-dark matter cosmic particles. Because dark matter interacts so weakly relative to normal particles, the proportion of dark particles that make it to the underground detector can be expected to be much greater than at the surface. Most of the light particles would have smashed into something along the way.
When particles reach the detector, they're confronted with a bath of liquid xenon. Should a dark matter particle ever so rarely interact with a xenon nucleon, the result should be recoil, a transfer of momentum. In this transfer of energy between incoming particle and nucleon, some of the energy is released as photons. This is light, or scintillation, and it's what the project looks for. This is all quantified generally as a scattering cross-section, or the probability of a nucleon-WIMP coupling even occurring.
Cross-sections are pretty confusing. A cross-section could also be thought of as a measure of the strength of an interaction, but in terms of two-dimensional area. The smaller the area, the smaller the strength, or the probability of an interaction. PandaX has now excluded cross-sections all the way down to 2.5×10−46. If WIMPs do exist, they sure aren't keen on interacting with our world.
There are other dark matter candidates beyond WIMPs, of course. Some of them aren't even so dark. The properties of WIMPs just happen fit really, really well into what we know about the universe. The PandaX experiment is still collecting data and is set for an upgrade to PandaX III this year, which will increase the amount of xenon in the detector from 500 kg to one ton.