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Though it has never been observed till now, we have strong evidences of the presence of dark matter in the Universe. Even more, dark matter is expected to constitute 85 % of the total mass of the Universe, and to be made of supersymmetric particles, called WIMPs (for Weakly Interacting Massive Particles), which would have no electromagnetic interaction with ordinary matter, and so remain invisible. In a first part of this talk, we will review different evidences we have about dark matter at all scales (Cosmic Microwave background, galaxy clusters, our Galaxy, etc.). Then we will investigate the different channels for the detection of dark matter (indirect, direct, in accelerators) and we will focus on the PICASSO experiment, which is a array of superheated droplet detectors (SDDs), located in the SNOlab underground laboratory in Sudbury, Ontario, Canada. PICASSO is designed for the direct search for WIMPs. Energy deposited during the passage of a particle through a drop of superheated liquid can induce bubble nucleation in the drop. The expansion of the bubble is accompanied by emission of an acoustic pulse which can be recorded by appropriate devices (piezoelectric sensors or microphones). By the study of the detection thresholds as a function of the temperature and the characterisation of the acoustic pulses, one can discriminate between the events generated by different types of particles, and isolate WIMP-like events. Upper limit derived from these studies may give constraints on the supersymmetric models of WIMPs. At the end of the talk, future upgrades of PICASSO will be presented. |