They're probably referring to Fritz Zwicky's observation in 1933 that the luminous matter in the Coma Cluster couldn't support the observed velocity dispersion.

They're probably referring to Fritz Zwicky's observation in 1933 that the luminous matter in the Coma Cluster couldn't support the observed velocity dispersion. Zwicky hypothesized that there was a large amount of non-luminous matter that allows the velocity dispersion to be as large as it is. Zwicky's hypothesis wasn't taken seriously until the 1970s when Vera Rubin and others found a similar effect in galactic rotation curves.