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Since the first decades of the past century the astronomers pointed out that the visible matter is not enough to explain the motion of the celestial bodies. In particular, in the ?30s, the Swiss astronomer Fritz Zwicky, while investigating the motion of the galaxies in the Coma cluster, noted it was much faster than expected; this could be explained only considering the galaxies embedded in a gravitational field much stronger than that induced only by the visible objects in the cluster. Few years later the observations of Zwicky were confirmed by observations on the Virgo cluster. In the ?70s several observations have shown that similar effects are present in the spiral galaxies too. In fact, in the galaxies as our one, the Milky Way, most of the visible matter is in the central core and ? if all the mass of the galaxy were the visible one ? the astrophysical objects would be expected to rotate around it with a velocity which should decrease with the increase of their distance from the centre of the galaxy. On the contrary, the observations have shown that these celestial bodies move much more quickly than expected, requiring the presence of a non-visible matter which increases the gravitational attraction. This matter is generally named Dark Matter.
Firstly, the Dark Matter was supposed to be made of invisible objects of strictly astrophysical nature: stars too weak to be detected, planets, black holes or neutron stars, etc. But various kinds of observations have excluded that they might be the solution of this puzzle. In particular, it is worth mentioning the studies made through the observation of the gravitational lensing: when one of these ?dark? objects move between the Earth and a far luminous star, the gravitational field of the invisible object deviates part of the emitted light towards the Earth, so that the luminosity of the distant star temporarily increases for the observer in a peculiar way. Similar events have been searched observing for example the stars in the Large Magellanic Cloud. Moreover, other results arise from the theories which describe the primordial nucleosynthesis, explaining processes of the light nuclei formation in the early Universe. In particular, they allow the calculation of the present abundance of light nuclei as hydrogen, helium and lithium, which are by far the most abundant elements in the Universe. On the other hand, the abundance of the light nuclei in the Universe has been measured and the observations satisfactorily agree with the theories. The results credit that the matter ? as we are accustomed to know it ? cannot be larger than 4% of the total mass of the Universe and that large part of the matter in the Universe is dark and cannot have the same nature as the atomic nuclei.
It is worth recalling that theories about modified gravity have also been hypothesized to explain the observed gravitational effects. However, these theories are lacking of generality and sometimes they require the presence of a certain amount of dark matter particles to reconcile all the observations and to account for the structure formation. Thus, they do not offer in the practice any suitable alternative explanation for the observed effects.
Further relevant information arises from the observations on the relic radiation from the Big Bang, which is named comic microwave background. The study of such a background radiation ? together with the measurements on the Supernovae Ia ? allows the investigation about the composition of the Universe in the framework of the Big Bang theory. The observations of recent years suggest that about 70% of the density of the Universe can be accounted by the so-called dark energy, which would give rise to an accelerated expansion of the Universe, and whose nature is at present very controversial. The further 30% is due to the ordinary matter and mainly to Dark Matter. The latter must therefore be composed of subatomic particles, presumably generated in the initial time of the Universe. The Dark Matter particles form a non-dissipative gas in galaxies like our Milky Way (passing, for example, continuing through the Earth), and constitute a halo that contributes to the gravitational attraction of the Galaxy.
The experimental techniques of the particle Physics can allow the investigation of the presence of Dark Matter in our galaxy. Therefore, the particle Physics, whose aim is to study the ultimate constituents of the matter at small scale, can help in explaining the formation and the motion of large celestial bodies and the overall evolution of the Universe.
A wide zoology of particles has been proposed in literature as Dark Matter candidates in a new physics beyond the so-called Standard Model of the particle Physics. The Standard Model is the theory used so far to explain the experimental observations on the fundamental constituents of matter. However, although able to describe many phenomena, this model has some unresolved issues that could be set in more general theories extending it to higher energy scale. The Dark Matter particles should have some distinctive features: i) they should be massive particles (in order to contribute to the gravitational attraction in galaxies and clusters, and to the total mass of the Universe); ii) their interaction probability with ordinary matter should be very small (so that they may have survived in large quantity up to now, although produced in the early Universe); iii) they should be neutral. Firstly, the neutrinos were considered as the Dark Matter particles, however, both the current knowledge about them and the necessity to suitably explain the formation of the galactic structures and clusters, restrict them to a marginal role (<1% of the total density). Some candidates are often grouped under the term WIMPs (Weakly Interacting Massive Particles), although they can have very different interaction type, mass value, etc. in the many different possible astrophysical, nuclear and particle physics scenarios. For example the neutralino, a particle foreseen in the supersymmetric theories, belongs to this class of candidates; it is worth noting that its expected mass can vary from the mass of the proton to thousands times more, depending on the adopted theoretical assumptions. Many other candidates are available in the literature; among them we recall the axions and the light bosons (which have a similar nature than that of axions, but higher mass ? of the order of keV); the latter ones can potentially also explain some particular astrophysical observations.
Various activities are carried out in underground laboratories to pursue a direct detection of Dark Matter particles; they use different target materials, different methodologies and have different sensitivity to the various candidates, interaction types and scenarios. Some activities are also carried out in space, underwater or under the polar ice, offering ? as by-product ? some strongly model dependent results. In fact, they perform indirect searches for Dark Matter particles trying to point out the presence (with respect to a background contribution calculated on the basis of some hypothesized model) of secondary particles produced (but just in case of some candidates and under some special assumptions) in processes of annihilation due to an accumulation of these particular candidates within the celestial bodies as the Sun or the Earth or the centre of the Galaxy.
In the following just arguments on the direct detection will be focused. This field of particle Physics requires the development and use of special experimental techniques able to point out the signal that the Dark Matter particles give, while interacting with ordinary matter. In fact, because of the very small interaction probability, these events are obviously rare and can be observed only if the number of events of different origin is suitably minimized. For this reason studies on the dark matter particles are carried out in underground laboratories, like the Gran Sasso, where the shielding offered by the mountain overlooking the laboratory (there about 1500 m of rock) absorbs most of the particles hitting the surface of Earth, the so-called cosmic rays. Similarly, to allow the detection of Dark Matter particles the radiation produced by natural and artificial sources of radiation that surround the experimental apparatus must be reduced. The detectors should be realized by highly radiopure materials and should be surrounded by very heavy multi-component shields formed e.g. from selected lead, copper, cadmium, paraffin, etc., that absorb the environmental radiation of the laboratory. As regards the detection techniques of Dark Matter particles, there are several approaches that have different sensitivity not only to different candidates, but also to the same type of candidate in different scenarios and can provide complementary results. The aims of these researches are: i) the experimental detection of the Dark Matter particles in the Universe independently on their nature, ii) the composition and nature of this component of the Dark Matter. The first point requires a model independent experimental approach for the detection, while the latter one always requires a model dependent comparison in several possible astrophysical, nuclear and particle Physics scenarios. Model independent approaches are primarily based on effects induced by the motion of the Earth. In particular, at present the well-feasible one, which is sensitive to many candidate particles with mass and cross sections in a broad range of values, is known as the Dark Matter annual modulation signature. It has successfully been and is exploited at the National Laboratories of the Gran Sasso by the DAMA set-ups: the former DAMA/NaI (about 100 kg of highly radiopure NaI(Tl) sensitive volume) and its successor DAMA/LIBRA, which consists of about 250 kg sensitive mass and is currently in data taking.
The DAMA/LIBRA experiment offers today in this field e.g. the largest exposed mass, the highest radiopurity, the full continuous control of the running condition, the largest collected exposure. It consists of 25 detectors made of highly radiopure NaI(Tl) detectors, for a total sensitive mass of about 250 kg. The crystals are inserted into a shield of several tonnes, consisting of different thicknesses of selected copper, lead, cadmium foils, polyethylene/paraffin and a cover of about one meter of concrete made by the same Gran Sasso rock. The detectors are also isolated from the air of the laboratory by three levels of sealing and continuously maintained in a high purity nitrogen atmosphere, avoiding any contact with the radioactive gas Radon present in traces in the air external to the installation.
The NaI(Tl) scintillators are particularly suited for such kind of experiments, because Sodium and Iodine are sensitive to dark matter particles of various nature, having either small or large mass, and interacting with matter by various different processes. When a Dark Matter particle interacts in a NaI(Tl) scintillator crystal, induces a series of processes whose final result is the emission of characteristic light, which can be observed. An important property of these NaI(Tl) detectors and of the whole set-up is the high intrinsic radio-purity (reached after a long work that involved for many years the experimental group and the manufacturing side). They, in contrast to materials used in common life, have an extremely small residual content of radioactive contaminations (in each DAMA NaI(Tl) crystal the number of radioactive nuclei is some orders of magnitude lower than a billionth of the number of Iodine and Sodium atoms).
The DAMA/NaI and DAMA/LIBRA set-ups have been designed and built to exploit the model independent annual modulation signature and to point out the presence of Dark Matter particles in the galactic halo independently on hypothesized theoretical models. This signature originally suggested in the mid ?80s by Freese et al. requires that many specific peculiarities are simultaneously satisfied in order to provide unequivocal evidence. In fact, as a consequence of its annual revolution around the Sun, which is moving in the Galaxy, the Earth should be crossed by a larger flux of Dark Matter particles around 2 June (when the Earth orbital velocity is summed to the Sun velocity in the Galaxy) and by a smaller one around 2 December (when the two velocities are opposite). Thus, this signature has a different origin and peculiarities than the seasons on the Earth and than effects correlated with seasons (consider the expected value of the phase as well as the other requirements listed below). The DM annual modulation signature is very distinctive since the effect induced by DM particles must simultaneously satisfy all the following requirements: the rate must contain a component modulated according to a cosine function (1) with one year period (2) and a phase that peaks roughly around about 2nd June (3); this modulation must only be found in a well-defined low energy range, where DM particle induced events can be present (4); it must apply only to those events in which just one detector of many actually ?fires? (single-hit events), since the DM particle multi-interaction probability is negligible (5); the modulation amplitude in the region of maximal sensitivity must be < 7% for usually adopted halo distributions (6), but it can be larger in case of some possible scenarios. This approach is very competitive and it needs the realization of an experiment with large mass, high radio-purity and high control of the running conditions, as is the case of the former DAMA/NaI and of the present DAMA/LIBRA experiments.
DAMA/NaI has collected data over seven annual cycles and has completed its data taking on July 2002. It observed an annual modulation effect that satisfies all the many peculiarities of the signature. This measurement was the first experimental evidence of direct detection of Dark Matter and of its presence in our Galaxy. This result was further and independently confirmed by the results obtained in the first 4 annual cycles of DAMA/LIBRA released in 2008 and by the results obtained in the following 2 annual cycles released in early 2010, and by the cumulative DAMA/LIBRA-phase1 data in 2013. In fact, DAMA/LIBRA has also measured an annual modulation effect with the same peculiarities of the signal observed by DAMA/NaI, reaching a better sensitivity thanks to the larger exposed mass. If the total cumulative data of the two experiments are considered, the measurements refer to 14 annual cycles. In each annual cycle an independent effect of this modulation has been observed. The cumulative exposure of about 1.33 tons x years is several orders of magnitude greater than the exposures typically achieved in this field. The observed modulation effect is highly significant. Neither systematic effects nor side reactions able to mimic the signature were found, no systematic effect able to explain the observed modulation amplitude and to satisfy simultaneously all the requirements of the signature has been found or suggested by anyone over more than a decade. The results obtained by DAMA/NaI and DAMA/LIBRA satisfy all the requirements of the signature and indicate the presence of dark matter particles in our galaxy with high significance. The observed effect is also consistent with a wide range of possible scenarios on the nature of these particles, on characteristics of interaction and on the structure of the galactic halo. In particular, many interpretations of the observed effect in terms of several possible candidates and of various scenarios of particle physics, nuclear physics and astrophysics have been performed so far; other interpretations have been proposed and/or planned. It is worth noting that no experiment whose results can be compared in a model independent way with those of DAMA/NaI and DAMA/LIBRA is available in the field. We also note the currently available model dependent negative results obtained in indirect and direct searches are not in robust conflict with the model independent result of DAMA, while compatibility exists with some possible recent positive hints.
The unique properties and characteristics of DAMA/LIBRA (whose first upgrade has been performed in September 2008, while the second more important one has been carried out in Fall 2010) will also allow in the future the further investigation of the features of the candidate particles and of the astrophysical, nuclear and elementary particle aspects related to the signal characteristics and of the possible second order effects.
Over the years the DAMA experiment has also performed other searches on many rare processes using its experimental set-ups. In fact, in addition to the mentioned DAMA/LIBRA and DAMA/NaI set-ups (which, in addition to the investigation of dark matter, have also allowed the study of other rare processes with data collected in various energy regions) other measurements have been performed using: i) the DAMA/LXe set-up, about 6.5 kg of liquid Xe free from Krypton and enriched in 136Xe or 129Xe (which was also the first dark matter experiment with liquid Xenon target); ii) the DAMA/R&D set-up a facility for small-scale experiments with scintillation detectors; iii) DAMA/Ge and other intrinsic low background germanium detectors in the Gran Sasso laboratories. Among the obtained results we remember, for example: the study of several double beta decay modes in many isotopes (e.g. Ca, 48Ca, 64Zn, 70Zn, 180W, 186W, 106Cd, 108Cd, 114Cd, 136Ce, 138Ce, 142Ce, 130Ba), of possible charge non-conserving processes, of possible processes that violate the Pauli exclusion principle, studies on the stability of the nucleon and electron, solar axions searches, searches for exotic particles, etc..