Galactic cosmic rays are thought to be accelerated in strong shock waves induced by supernova explosions sweeping across the interstellar medium. But the phase of the interstellar medium from which cosmic rays are extracted has remained elusive until now. By studying the different locations and mechanisms of acceleration of these particles in our Galaxy, and by using recent measurements of the cosmic ray composition by the AMS-02, Voyager 1 and SuperTIGER experiments, we have obtained new information on the origin of these cosmic particles: they are most likely accelerated in superbubbles of plasma at millions of degrees that form around massive star clusters.