Axions are another well-motivated dark matter candidate. While axions are much lighter than the SUSY relics discussed in the previous section and are produced by a very different mechanism, they are indistinguishable to theoretical cosmologists studying galaxy formation and the origin of large scale structure. Both axions and SUSY relics behave as cold dark matter (CDM) and cluster effectively to form galaxies and large-scale structure. (See Steinhardt's and Ostriker's articles on structure formation).
Axions were proposed to explain the lack of CP violation in the strong interaction [74,75]. They are associated with a new U(1) symmetry: the Peccei-Quinn symmetry . As originally proposed, axions interacted strongly with matter. When experimental searches failed to detect axions, new models were proposed that evaded experimental limits and had the interesting consequence of predicting a potential dark matter candidate [39,57,77,22].
In the early universe, axions can be produced through two very distinct mechanisms. At the QCD phase transition, the transition at which free quarks where bound into hadrons, a bose condensate of axions form and these very cold particles would naturally behave as cold dark matter. Axions can also be produced through the decay of strings formed at the Peccei-Quinn phase transition [19,20]. Unless inflation occurs after the P-Q phase transition, string emission is thought the dominant mechanism for axion production. While Sikivie and collaborators  has argued that Davis and Shellard overestimated string axion production, recent analysis  confirm that strings are likely to be the dominant source of axions. Axionic strings will not produce an interesting level of density fluctuations as their predicted mass per unit length is far too small to be cosmologically interesting.
The properties of the axion are basically set by its mass, , which is inversely proportional to the scale of Peccei-Quinn symmetry breaking, . The smaller the axion mass, the more weakly the axion is coupled to protons and electrons. Raffelt  reviews the astrophysical arguments that imply eV. If the axion had a larger mass, then it would have had observable effects on stellar evolution and on the dynamics of SN 1987A. If we require that the energy density in axions not ``overclose'' the universe, then implies that eV. If strings play an important role in axion production, then the cosmological limit lies closer to and there is only a narrow window for the axion model .
Axions are potentially detectable through their weak coupling to electromagnetism . In the presence of a strong magnetic field, the axionic dark matter could resonantly decay into two photons. The first generation of detectors consisted of experiments in Florida and at BNL that looked for this decay in a tunable resonant cavity. Since the Peccei-Quinn scale is not well determined, these experiments have to scan a wide range of frequencies in their search for the axion. These experiments were an important first step towards probing an interesting region of parameter space.
In the past few years, the search for axions has been revived by two new experimental efforts. Karl von Bibber  and his group at LLNL have built a cryogenically cooled cavity; this detector should be able to reach into cosmologically interesting region of parameter space.In Kyoto, Matsuki  and his group plan to use an atomic beam of Rydberg atoms as an axion detector. This detector would detect an axion in the galactic halo through its excitation of a Rydberg atom in the n-th energy state to the n+1 energy state. The Kyoto collaboration also promises to probe the cosmologically interesting region of parameter space.