The most promising way around the direct growth hindering m-sized barrier is in-situ planetesimal formation via gravitational collapse of massive particle clouds. In this talk, I present the circumstances under which planetesimals, which we believe are the same as cometesimals, and its remnants we today call asteroids, are thought to form and how we were able to close the missing link between mm and km sized objects. For the presented work, I performed hydro-simulations of the so called streaming instability within the environment that a collapse is expected to happen in. This instability is known mainly for its ability to concentrate dust, but I will introduce its turbulent diffusive behavior as the limiting factor for this in-situ collapse to occur. We further find this diffusive process to be pre-determining the planetesimal birth size of around 100 km independent of semi-major axis! With this we were the first to find an explanation why observed asteroid and classical Kuiper-belt size distribution, as well as Jupiter Trojans sizes and Pluto impact crater statistics, have a kink exactly at this size precise size. Moreover, we could derive from first principles a collapse criterion, similar to the Jeans criterion for stars but for planetesimals, giving us a prediction on the initial planetesimal size and find this in agreement with our simulations.