<human>:  Who discovered dark matter?
<bot>: Dark matter was first postulated by Fritz Zwicky in 1933, but it wasn’t discovered until the 1960s. Zwicky was a Swiss astronomer who noticed that the velocities of galaxies in clusters of galaxies were not what they should be if the cluster was gravitationally bound. He concluded that there must be some unseen mass in the clusters that was causing the galaxies to fly away from each other. This unseen mass is called dark matter.