I will discuss the mass discrepancy phenomenon in galaxies usually accounted by postulating the presence of a non luminous component. In the theoretical framework of Newtonian gravity and Dark matter halos I will recall the properties of the latter as emerging from the state-of-the-art of numerical simulations performed in the current LCDM scenario of cosmological structure formation. I will then describe the phenomenology of the distribution of dark matter in spirals, ellipticals, and dwarf spheroidals. Care will be given to show that such a coherent observational framework is obtained from different and large samples of galaxies and by means of very different methods of investigation and by exploiting different tracers of the gravitational field. I will then highlight evidence that the distribution of dark and luminous matter are closely correlated.
