In the talk, I review some recent developments on the topic of self-interacting Dark Matter (SIDM), paying particular attention to restrictions on the SIDM (total) cross section from the use of novel observables in merging galactic structures, as well as the r\^ole of SIDM on the milky way halo and its central region. In this latter context, I discuss interesting particle-physics inspired SIDM models involving self-interacting right-handed (Majorana) neutrino DM (with mass of a few tens of keV), that may exist in minimal extensions of the Standard Model. I also describe briefly the phenomenology of these models, and argue that they can tackle the basic problems of
the ``small-scale Cosmology crisis''.