Fritz Zwicky:

Fritz Zwicky was born in Varna, Bulgaria in 1898. Much like Oort was studying the movement of stars within the Milky Way, Zwicky was studying the movement of galaxies in the universe. To be more specific, he was studying a cluster of galaxies known as the Coma Cluster. Using Newton's laws, he was able to measure the mass of the cluster.

After calculating the mass of the cluster, he measured the total light emission of the cluster. Knowing the light output and mass of the Kapteyn stellar system, he calculated the ratio of total light output (which is a measurement of energy), to their mass (which is used to calculate potential and kenetic energies). These light to mass ratios should be constant throughout the universe and, until then, they were. What he found was a ratio in a Kapteyn star was larger by more than 100%. Not knowing what could account for this inaccuracy, he concluded that the Coma Cluster must have matter that can't be measured by using light. This is where we get the term "Dark Matter". More on this study can be found here.

 

Find more on Zwicky's life here.