Current cosmological data imply the existence of non-baryonic dark matter. We have discussed some of the most popular candidates and shown that none of the candidates known to exist, i.e. the active neutrinos, can be non-baryonic cold dark matter. Hence to explain the nature of cold dark matter we need to invoke hypothetical particles that have not been detected yet. Some of these hypothetical particles have been suggested for reasons different than the dark matter problem (such as sterile neutrinos, neutralinos, and axions), some others have been proposed mainly as a solution to the cold dark matter problem (e.g., self-interacting dark matter, WIMPZILLAs, etc.). Although most studies focus on the first category of candidates, especially neutralinos and axions, we should keep an open mind.
To illustrate how we can find out if dark matter is made of elementary particles, we have used neutralino dark matter as our guinea pig to survey several methods to search for non-baryonic dark matter. These methods range from a direct detection of dark matter particles in the laboratory to indirect observation of their annihilation products produced in the core of the Sun or of the Earth and in galactic halos, including our own. Direct searches may have found a signal from WIMPs (the annual modulation), but this claim is highly controversial at the moment. Future direct searches have great promise, and might even be able to explore the local velocity distribution of WIMPs. These searches are complemented by indirect searches for high-energy neutrinos from the core of the Sun or of the Earth. Indirect searches using gamma-rays and cosmic rays from annihilations in the galactic halo are subject to uncertainties related to the detailed structure of the dark matter halo. Even more so are predictions for dark matter signals from the Galactic Center. Despite this, some anomalies in cosmic ray fluxes, namely a positron excess, may be explained by neutralino annihilation, and future gamma-ray observations may discover a gamma-ray line from neutralino annihilation in our galactic halo.
All of the examples we have presented are without doubt simple, elegant, and compelling explanations for the nature of non-baryonic dark matter. As we ponder on which one of them is realized in Nature, we must remember the words of astrophysicist Thomas Gold (as quoted by Rocky Kolb): "For every complex natural phenomenon there is a simple, elegant, compelling, wrong explanation."