Nearly a century of compelling astronomical observations has indicated that the visible and ordinary matter constitute only a small fraction of the Universe. A large part of the Universe is indeed expected to be in form of non-visible matter, Dark Matter, and of a mysterious energy component, Dark Energy.
The measurements on the anisotropies of the Cosmic Microwave Background strongly suggest that the Universe is flat, having a density equal to the critical one (ΩTOT = ρTOT/ρcrit =1, with ρcrit =3H2/8πG about 6 hydrogen atoms per m3). Cosmological models through the CMB measurements, the observational data of the Supernovae IA, the Baryonic Acoustic Oscillation and the large-scale structures, allow one to estimate the relative fraction of all the matter and energy components in the Universe. They can be expressed in terms of their density values, ρx, in unit of critical density (Ωx = ρx/ρcrit). The obtained values are: Ωrad ≈ 5 × 10-5 for the radiation density, Ωb ≈ 0.05 for the baryonic matter, ΩDM ≈ 0.27 for the non-baryonic Dark Matter, Ωλ ≈ 0.68 for the Dark Energy. Regarding the neutrino content of the Universe, the neutrino density is strongly constrained by the large-scale structure of the Universe and a value below Ων ≈ 0.01 is obtained. The estimated baryonic matter contribution, Ωb ≈ 0.05, is also supported by the Big-Bang nucleosynthesis (BBN) that predicts the abundances of the light elements in the Universe. This value is much larger than the cosmic density of the luminous matter that is only, Ωlum ≈ 0.004, suggesting that most baryons are dark and, probably, in the form of a diffuse intergalactic medium. Thus the values of the cosmological parameters support that most of the matter in the Universe is Dark, i.e. do not emit or adsorb light, in form of relic particles from Big Bang with non-baryonic nature.
From the astronomical point of view, many observations gave evidence of the existence of a non-luminous matter component in the Universe at very different scales.
The first evidence dates back to 1933 when F. Zwicky, measuring the velocity dispersion of the galaxies in the COMA cluster, realized that these galaxies move too fast to be bound to the system if the only visible matter components contribute to the gravitational field of the cluster. A large part of the non-visible matter needs to be present in the cluster to explain the gravitational attraction felt by the galaxies.
A crucial step for the existence of Dark Matter in the Universe was then performed in 1970’s when its presence inside spiral galaxies was pointed out. It was observed that the rotational velocity in the galactic plane doesn’t decrease outside the halo of visible matter — rather it stays roughly constant. This implies the existence of a Dark halo that contributes to the mass of the galaxy proportionally to its radius. The amount of non-luminous matter is estimated to be about four times larger than luminous one.
Other evidence of the existence of the Dark Matter has been obtained from the observation of X-ray emitting gases surrounding elliptical galaxies, the velocity distribution of hot intergalactic plasma in clusters, and weak gravitational lensing. All these observations are based on gravitational effects and lead to the same conclusion on the prominent presence of Dark Matter in the universe. Most recently a persuasive result was obtained in 2004 by the observation of the “Bullet Cluster” (1E0657-558): a collision between two clusters of galaxies in which the hot baryonic matter component of the two clusters - observed by the X-ray radiation - collided and decelerated, remaining in the central part of the system, while the mass distribution was concentrated in the outer part of the system (as revealed by the weak lensing.) This displacement of ordinary matter with respect to the obtained mass profile supports that in the cluster the largest matter component is collisionless and has non-luminous nature. The “Bullet Cluster” can be explained only by considering the existence of non-baryonic Dark Matter in the clusters and, moreover, cannot be interpreted in terms of theories of modified gravity.
The Milky-Way is also embedded in a large dark halo whose density in the solar neighborhood has been estimated to be of order of ≈ 0.3 GeV/cm3, depending on the halo model.
All the evidence supports the presence of Dark Matter in the universe as an important ingredient for its evolution, playing a crucial role in the structure formation. Therefore, a large component of the Universe is expected in form of non-baryonic relic particles from the Big Bang. These particles, to survive with a sizeable abundance up to the present epoch, have to be neutral, stable or with a lifetime comparable with the age of the Universe and have very low interaction rate with ordinary matter. It is remarkable that no particle belonging to the Standard Model of particle physics can be a good Dark Matter candidate; this can be considered as a motivation for theories extending the Standard Model in which, indeed, many candidates having different nature and interaction types have been proposed. Among the many Dark Matter candidates are stable particles proposed in SUSY theories (as e.g. neutralino or sneutrino in various scenarios), inelastic Dark Matter, electron interacting Dark Matter, sterile neutrino, Kaluza-Klein particles, self-interacting Dark Matter, axions or axion-like (light pseudoscalar and scalar candidate), mirror Dark Matter, etc. Moreover it is important to note that, considering the richness of the ordinary particles in the visible Universe, one could also expect that the Dark Matter particles may be multicomponent.
A widely considered interaction process of Dark Matter particle with ordinary matter is the elastic coherent scattering; in this case an experiment - with the aim of detecting Dark Matter interactions with target nuclei - would measure the nucleus recoil energy; anyhow, other detection processes can be considered for various Dark Matter candidates in which the interaction signal is in total or in part of electromagnetic nature. Since the velocity of the particle is of the order of around 300 km/s, and considering the detector-quenching factor for nuclear recoils, the energy released in the interaction is expected in the keV energy range. Moreover the interactions are also predicted to be rare because of the very low cross section and thus the experiments investigating Dark Matter direct detection - to be sensitive to the expected signal with respect to background - exploit low background techniques. Firstly, they are carried out in underground laboratories where the radiation produced by the cosmic ray is highly suppressed. To further reduce the measured background radiation, the detectors are placed inside massive shield able to adsorb the external environmental radiation; in addition all the employed materials, as well as the detector itself, are characterized by a very low content of residual radioactive isotopes. Various experimental solutions and detector configurations have been developed specifically by each experiment to further reduce residual background. The experiments carried out in the Gran Sasso laboratory use different target materials and exploit different approaches. A model independent approach, based on the study of the time dependence of the Dark Matter signal, the so called Dark Matter annual modulation signature is exploited; this approach has the merit to be model independent (i.e. no hypothesis on the Dark Matter nature and interaction is needed) and to require the satisfaction of many specific peculiarities offering an unambiguous signature for Dark Matter. On the other hand other approaches are followed by various experiments in which hypothesis on the nature of the Dark Matter particle interaction are required. This is the case of experiments that apply rejection procedures of the electromagnetic rate, assuming the Dark Matter signal to be present only as nuclear recoils.