- The virial theorem applied to the Coma cluster by Fritz Zwicky in 1933. In the paper “Die Rotverschiebung von extragalaktischen Nebeln”, Helvetica Physica Acta 6: 110–127 (1933), Zwicky studied the motion of galaxies in the Coma cluster and concluded that they were moving too fast to be bound together unless there was a large amount of unseen matter in the cluster adding to its gravitational strength. Zwicky named this matter “Dunkle Materie” or, literally translated, “Dark Matter”.
- Zwicky’s work on dark matter did not have a very large scientific impact until 1970 when Vera Rubin studied the rotational curve of the Andromeda galaxy, “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions”, Astrophys.J. 159: 379 (1970), that the dark matter story really began. Just like Zwicky had discovered that the Coma cluster would need a significant amount of invisible matter to hold together, Rubin found that the stars in the Andromeda galaxy were moving too fast to be gravitationally bound unless there was a large amount of dark matter.
The history of the discovery of dark matter therefore seems rather set in stone, Zwicky was the first to discover it in 1933 and it gained traction after the work of Rubin in 1970. However, there is one person who has been forgotten in this storyline, professor Knut Lundmark. Apart from discovering the expansion of the Universe before Edwin Hubble (Nature 490, 176 (2012)), it turns out that Lundmark also had a thing or two to say about dark matter, something which would be forgotten, left to collect dust in a drawer, rediscovered by someone else, and only 85 years later found again.
In a conference celebrating the 40-year anniversary of the German Astronomical Society in 1930, Lundmark presented his findings about the velocities of stars within galaxies. Sounds familiar? It should. What Lundmark had done was essentially to measure the galaxy rotation curves of several different galaxies and compared the mass required to the luminous mass of the galaxies. His conclusion was the same as that of Rubin 40 years later, a large part of the mass of a galaxy is in the form which is not visible to us. Like Zwicky would do three years later, Lundmark spoke about this additional mass as “Dunkle Materie” although he never went as far as suggesting that it would be a new form of matter, only that it was matter which we could not see. Lundmark’s findings were published in Lund Medd. 125, 1 (1930) and subsequently seems to have been largely forgotten.
The rest, as they say, is history. With Zwicky’s and Rubin’s results becoming highly popular and the essential foundation for our modern understanding of dark matter, Lundmark’s work was not rediscovered until 2015. On april 17 of that year, Lars Bergström, professor in theoretical physics at Stockholm University, gave a presentation at a conference in Trieste, Italy, where he told the story of how he had stumbled upon the reference to Lundmark’s work.
As to the sensmoral of the story, dark matter was eventually discovered, but it is not always the first person to discover something who history will credit.