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Introduction: Basics of Non-Baryonic Dark Matter

There is overwhelming evidence that most of the matter of the universe is dark and a compelling motivation to believe that it is mainly of non-baryonic origin. The matter density of the universe, $\rho$, is currently expressed in terms of its critical density with , where H0=100.h.km-1. Mpc-1 with 0.4 < h < 1. Measurements and estimates of have been made at various scales by a diversity of methods. The visible stars account only for a small fraction of (0.002 ). As the scale of the observed cosmic structures increase, the resulting values of become larger, approaching the unity value predicted by inflationary cosmologies. Big-bang nucleosynthesis constrains the baryon fraction of to be within and , and so baryonic dark matter is needed. On the other hand, the large values of at increasing scales together with the smallness of imply that exotic, non-baryonic particle dark matter should be the main component of the universe. Extensions of the Standard Model of Particle Physics provide non-baryonic candidates to dark matter. From the cosmological point of view two big categories of such candidates have been proposed: Cold Dark Matter (CDM) and Hot Dark Matter (HDM) according to whether they were slow or fast moving at the time of the galaxy formation. Their relative proportion is so as to properly generate the observed cosmic structures by gravitational evolution of the scale-invariant primordial density fluctuation. The simple CDM model needs to be mixed with a small fraction of HDM to match the observed spectral power at all scales. The mixed model featuring (h =0.5, ) is one favoured option. Recent values of the Hubble constant ( ) might favour however a Cold DM model with a non-zero cosmological constant and . The typical HDM candidates are neutrinos of a few eV, which could provide the right critical density. The most likely candidate is the tau neutrino $\nu_\tau$. There is no known method proposed so far to directly detect the hot DM relic neutrinos and so terrestrial sources are used to explore this possibility. The discovery of a $\nu_\tau$ mass in the few eV range would favour this form of DM and so several neutrino oscillation experiments are under way to look for such issue. In the CMD sector, typical candidates are heavy Dirac or Majorana neutrinos in the GeV-TeV mass range or other heavy, weakly interacting neutral particles, generically called WIMPs. A distinguished Majorana WIMP is the neutralino -the lightest (and stable) supersymmetric particle of SUSY theories. Accelerator results constrain the neutralino mass, for representative values of its parameter space, to be above 10-20 GeV. Neutralino relic abundances of cosmological interest are in the range . Another celebrated CDM candidate is the axion, a non-thermal relic invented to solve the strong CP problem. 
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