only weakly, mainly via the gravitational force. Computer simulationsof the formation of large-scale structure also provide valuable informationabout the behavior of the dark matter, which plays an important rolein shaping the structure. These studies show that the non-baryonicdark matter candidates can be divided into two categories dependingon the velocity with which the particles were moving when the universebecame dominated by matter (see pp. 5-8). During this epoch, a rapidlymoving particle (e.g., because its mass is small) is considered hotdark matter; a slowly moving particle is considered cold dark matter.Currently, the cold dark matter candidates, or a mixture of hot andcold, give the best agreement between computer simulation resultsand the observed large-scale structure.
Most cosmologists believe that the unknown matter needed to explainthe “missing mass” exists in the form of some yet-undetected elementary particle—a particle that is fundamentally different from ordinary matter.Such a particle would be a relic of some process in the high-energy-physicsera, but whether from the grand unification era or some later erais not known. Clearly, it is important to identify this non-baryonicdark matter, by direct searches and by accelerator experiments, withparticle theory providing guidance to focus the experiments. Chiefamong the theoretical elementary-particle candidates for non-baryonicdark matter are weakly interacting massive particles (WIMPs), axions,and neutrinos with finite mass. Of these, only neutrinos are knownto exist, but they are usually assumed to have zero mass. The experimentalupper limit for the electron neutrino mass is about 7 electron-volts(eV; the mass of the electron is about 511,000 eV). A sea of primordialneutrinos with this mass would provide sufficient dark mass to makeΩ = 1. However, neutrino dark matter would be hot and so does notwork well by itself in computer simulations of the observed large-scalestructure. Another class of phenomena from particle physics, calledcosmic strings or textures, can be added to act as seeds for cosmicstructure, or some cold dark matter (e.g., axions) can be added tomake the results look more like the observations.
The most likely dark matter candidates from particle theory are addedto a mix of ordinary matter and thermal radiation in a gigantic computermodel that simulates the complex physics of an expanding universethat contains collapsing clumps of matter. These simulations arecomplex and difficult to do. The goal is to find a set of parametersand values that produces a simulation with clumps that have a large-scalestructure much like that seen by astronomers. Another approach toidentifying the dark matter particles is to search for them directlywith techniques drawn from experimental particle physics.
There are good theoretical and experimental reasons to suspect thata new symmetry exists in nature, known as supersymmetry, which mightenable gravity to be unified with the weak, electromagnetic, andstrong forces. If supersymmetry exists, then every fundamental particleof ordinary matter and radiation has a supersymmetric partner particle,as yet undetected. The lightest supersymmetric particle cannot decay(because there are no lighter particles to decay into) and wouldtherefore have survived from the time of the early universe untilnow. Such a particle's interactions with ordinary matter would bevery weak, and current accelerator experiments tell us that the massof the lightest supersymmetric particle is greater than 20 GeV (billionelectron-volts; the mass of the proton is about 1 GeV)—massive foran elementary particle. Thus WIMPs make an ideal candidate for non-baryonicdark matter in the universe. They are imagined to be only weaklyassociated with luminous matter, for example, forming a loosely boundhalo around our galaxy and others.
Laboratory detectors are now under construction in several countriesto look for a flux of WIMPs with mass in the range from 5 to 100GeV. Early results from conventional detectors have already set usefullimits on the flux of WIMPs, and new efforts are starting