Seeing the Unseeable — Part II
Dark matter should be streaming through our planet as it travels through the galaxy. On rare occasions, a WIMP will bump into an atomic nucleus and cause it to recoil, just as a pool ball does when struck by the cue ball. The predicted recoil energies are almost imperceptible but may be within the range of sensitive detectors. Cryogenic technology slows the natural vibrations of atoms and makes it easier to notice any recoil. The energy deposited in the detector holds the key to pinning down the fundamental properties of dark matter. Two experiments, DAMA and CoGeNT, have claimed to detect a signal, but others, such as XENON and CDMS, have found nothing. These and other new experiments are improving their sensitivities rapidly, promising an exciting near future for this field.
From WIMPS to galaxy clusters
The WIMP Coincidence
Given the expected mass of WIMPs and the strength of their interactions, which govern how often they annihilate one another, physicists can easily calculate how many WIMPs should be left over. Rather amazingly, the number matches the number required to account for cosmic dark matter today, within the precision of the mass and interaction-strength estimates. This remarkable agreement is known as the WIMP coincidence. Thus, particles motivated by a century-old puzzle in particle physics beautifully explain cosmological observations.

This line of evidence, too, indicates that WIMPs are inert. A quick calculation predicts that nearly one billion of these particles have passed through your body since you started reading this article, and unless you are extraordinarily lucky, none has had any discernible effect. Over the course of a year you might expect just one of the WIMPs to scatter off the atomic nuclei in your cells and deposit some meager amount of energy. To have any hope of detecting such events, physicists set their particle detectors to monitor large volumes of liquid or other material for long periods. Astronomers also look for bursts of radiation in the galaxy that mark the rare collision and annihilation of orbiting WIMPs. A third way to find WIMPs is to try to synthesize them in terrestrial experiments.
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Like any other breed of particle, WIMPs would have been produced in the fury of the big bang. High-energy particle collisions back then both created and destroyed WIMPs, allowing a certain number of them to exist at any given moment. But as the primordial soup cooled, the amount of energy available to create WIMPs diminished their number. This reduced the frequency of collisions until they effectively ceased to occur. At that point, about 10 nanoseconds after the big bang, the number of WIMPs became frozen in.