neutralino and WIMP dark-matter candidates

Something invisible outweighs all the stars and gas in the universe by about five to one, and we have no idea what it is made of. The most popular guess for decades has been a heavy particle that ignores light and almost ignores ordinary matter, drifting through the galaxy like a ghost while its gravity still herds the stars. A particle like that, with a mass roughly in the range of atomic nuclei but interacting only through the weak force, is called a WIMP — a weakly interacting massive particle.

The neutralino is the most studied specific candidate to play this role. In supersymmetry, several electrically neutral superpartners — the photino, the zino, and two higgsinos — mix together, and the lightest resulting blend is the neutralino. If the lightest superpartner is stable (which happens under a symmetry called R-parity), it cannot decay into anything lighter, so neutralinos made in the hot early universe would still be around today. Remarkably, a WIMP with weak-force-scale interactions naturally freezes out of the early universe in about the right abundance to be the dark matter — a coincidence nicknamed the 'WIMP miracle'.

WIMPs are searched for three ways at once: made directly in colliders (seen as missing energy), caught recoiling off atomic nuclei in deep underground detectors, and spotted indirectly through the particles released when two WIMPs annihilate in space. The candid status is that all three searches have come up empty across the most natural WIMP mass range, and the limits are now extremely tight. This has not killed the WIMP idea, but it has shifted attention toward alternatives such as the axion and lighter hidden-sector particles.