The Search for Dark Matter

How can we determine the identity of dark matter even though it is invisible to our telescopes? Three main techniques are currently underway at facilities around the world.

Direct Detection

If dark matter particles permeate our galaxy, then they also should be here on earth. In fact, countless may have passed through your body while you read this sentence! They would almost never leave any trace of their passage, but every once in a while their interactions with ordinary matter might be detectable.

If WIMPs comprise the dark matter, every once in a while one of them should bump into an atom, knocking its nucleus out of place. The rate of this can be calculated from the specific WIMP model, but could range from once a day to less than once per century per kilogram of target mass. Several groups around the world have developed sophisticated detectors capable of picking out these extremely rare events. CDMS is one such experiment - see "The Science of CDMS" at left for more details!

Axions may also be detector directly, though using very different techniques. In this case the trick is to build a sort of echo chamber for radio waves in the presence of a large magnetic field. Passing axions can interact with the magnetic field, inducing a detectable change in the energy content of the chamber. Experiments such as ADMX are underway to test this possible source for dark matter.

Indirect Detection

The annihilation rate of WIMPs is crucial to determining the amount of dark matter left over from the early universe. WIMP annihilation could continue to the present day in areas of high WIMP density such as the centers of galaxies. Gamma ray and cosmic ray telescopes search the skies for unusual radiation that could be a result of this annihilation.

It's also possible for WIMPs to become captured by the gravity in the center of the earth or sun, producing substantial annihilation in our own cosmic backyard. Gamma rays won't escape from the center of these bodies, but neutrino telescopes such as Super-Kamiokande look for excess neutrinos from this annihilation.

Accelerator Production

If dark matter particles were produced in the early universe, it may also be possible to produce them at particle accelerators. Accelerators may also produce other particles related to the dark matter particle, which can help understand the physics behind dark matter. Extensive searches have been conducted for new particles of many kinds at LEP and the Tevatron, so farr without detection. The upcoming LHC (and proposed ILC) will extend these searches in upcoming years.