When was dark matter therolized?

Dark matter was first theorized in the early 1930s, primarily by Swiss astronomer Fritz Zwicky. He observed the Coma Cluster of galaxies and noticed that the visible matter could not account for the gravitational effects observed, leading him to propose the existence of unseen matter that he termed "dunkle Materie," or dark matter. This concept gained further support in the following decades as more evidence accumulated from various astronomical observations.