When was the first measurement of dark matter in a galaxy cluster reported?

Fritz Zwicky reported evidence for missing mass in the Coma Cluster in 1933. By comparing how fast the galaxies in the cluster were moving with the visible mass, he concluded that there had to be much more matter than could be seen. This was one of the earliest modern arguments for dark matter. It took decades for the idea to gain broad acceptance, but Zwicky's work is now seen as a landmark.