Intermediate mass black holes (IMBH) should be linking observed stellar black holes and supermassive black holes, but their formation mechanisms are still debated. Young and dense massive star clusters are promising environments for the formation of such black holes through collisions. An international team lead by MPA researchers has presented novel realistic simulations of star clusters, where these missing links form by rapid collisions of stars and black holes. The study also predicts an IMBH formation channel by the merging of black holes in a mass regime, which is excluded by stellar evolution models. In this “mass gap” a black hole merger has indeed been observed recently by the LIGO/Virgo gravitational wave collaboration.