> While some point to galaxy cluster work by Fritz Zwicky in the 1930s, dark matter was truly “discovered” in the 1970s by Vera Rubin, who was studying the rota

> While some point to galaxy cluster work by Fritz Zwicky in the 1930s, dark matter was truly “discovered” in the 1970s by Vera Rubin, who was studying the rotation of spiral galaxies. Rubin found that galaxies were spinning too fast for the matter we could see in them, yet they weren’t flying apart. And Zwicky found that galaxies were moving too fast for the matter we see in clusters yet the clusters weren't flying apart. This seems exactly analagous to Rubin's discovery so it is not clear why the author considers it not the true "discovery".