Participating in the "Dark Matter awareness week", a worldwide effort for disseminating specific information about Dark Matter, I will discuss the details of 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, we start by recalling the properties of the latter as emerging from the state-of-the-art numerical simulations performed in the current LCDM scenario of cosmological structure formation. We then report the complex and much-telling phenomenology of the distribution of dark matter in spirals, ellipticals, and dwarf spheroidals. We will then highlight the impressive evidence that the distribution of dark and luminous matter are closely correlated, and finally point out some shortcomings of the model.