On February 14, 1898, Swiss astronomer Fritz Zwicky was born. He is best known for his proposal of he existence of dark matter and counts as one of the most important astronomers of the 20th century.
Fritz Zwicky attended a grammar school in Zurich, Switzerland and enrolled at the ETH Zurich in order to study physics and mathematics afterwards. Zwicky became a research assistant around 1922 and worked at the California Institute of Technology in Pasadena starting from 1925. He received a scholarship by the Rockefeller Foundation. In California, he worked as a theoretical physicist and later astrophysicist at the observatories of Mt.Wilson and Palomar. 
He gained attention around 1933 when he coined the term ‘supernova‘ and theorized that they were most likely the transition of normal stars into neutron stars. In order to prove his hypthesis, Fritz Zwicky began hunting for supernovae. By the later 1930s, he is believed to have discovered more than a dozen of them. In his whole life, it is assumed that Zwicky discovered more than 120 supernovae. During World War II, Fritz Zwicky began to get active in Pasadena in rocket engineering. Also, he was known to establish a program for academic libraries that suffered from the war. In the later 1940s, Zwicky became the scientific director of the company Aerojet and worked on the improvement of engines, which resulted in several patents. 
Fritz Zwicky is by many remembered as the father of Dark Matter even though this has been a topic of discussion. Zwicky first recognized that in rich clusters of galaxies, a large portion of the matter is not visible. In 1933, he published a scientific work, estimating that “the total mass of the COMA cluster of galaxies from the motions of the galaxies within that cluster. Using the virial theorem he came to the conclusion that the galaxies were on average moving too fast for the COMA cluster to be held together only by the mass of the visible matter:
“In order to receive an average Doppler effect of 1000 km/s or more, which is what we have observed, the average density in the COMA system would have to be at least 400 times greater than that of visible matter. If this can be shown to be the case, then it would have the surprising result that dark matter is present in the Universe in far greater density than visible matter.”
It has been argued that Fritz Zwicky’s analysis was deduced from limited statistics, but still, his estimation results were considered reasonable. Zwicky’s estimates were off by more than an order of magnitude, mainly due to an obsolete value of the Hubble constant. The same calculation done today shows a smaller factor than the one Zwicky proposed back in the 1930s, based on greater values for the mass of luminous material. However, it is still clear that the great majority of matter appears to be dark. 
During his lifetime, Fritz Zwicky was known to be a great speaker and was invited for talks numerous times. He also published a tremendous amount of scientific works. More than 500 publications were made by Zwicky including his ‘Morphological Astronomy‘ from 1957 and he was honored for his contributions to astronomy around the globe. In 1972, Fritz Zwicky received the gold medal by the Royal Astronomical Society in London. [1,2]
At yovisto academic video search you may learn more about ‘The Fifth Element: Astronomical Evidence for Black Holes, Dark Matter, and Dark Energy‘ in a lecture by Professor Scott Tremaine.
References and Further Reading:
-  Fritz Zwicky’s Biography at the Zwicky Foundation
-  Fritz Zwicky: An Extraordinary Astrophysicist
-  The Hale Telescope at Palomar Observatory, SciHi Blog, January 26, 2014.
-  The Universe goes beyond the Milky Way – Edwin Hubble contributions to Astronomy, SciHi Blog, November 20, 2013.
-  Fritz Zwicky at Wikidata