Georg Raffelt, Max-Planck-Institut für Physik, München, Germany Escuela Avanzada de Verano, 7-11 July 2008, Cinvestav, Mexico-City Structure Formation in the Universe SmoothStructured Structure forms by Structure forms by gravitational instability gravitational instability of primordial of primordial density fluctuations density fluctuations A fraction of hot dark matter A fraction of hot dark matter suppresses small-scale structure suppresses small-scale structure