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Seeing the invisible
contained luminous mass alone, the velocities of the individual galaxies should have sent them flying to the far reaches of the Universe aeons ago. This was obviously not the case, and so something was exerting enough gravity to hold the cluster together. But what was astonishing ± so much so that it took decades for the astronomical community to fully accept Zwicky's result ± was the amount of matter involved. Whereas Oort had surprised everyone by saying that the Galaxy contained perhaps twice as much matter as could be seen in the form of stars, Zwicky's results revealed a far higher discrepancy between luminous and gravitational mass. According to his calculations, the Coma Cluster contained as much as fifty times the luminous mass, in some unseen form. The Coma Cluster was saturated with dark matter. Zwicky did all this in 1933, and three years later Sinclair Smith, at Mount Wilson Observatory, carried out a similar experiment with the Virgo Cluster. This is a much closer cluster of galaxies, and is irregular in shape. So massive and close is the Virgo Cluster that our Local Group of galaxies is being pulled towards it. (The Local Group of galaxies consists of two large spirals ± the Milky Way and the Andromeda galaxy ± along with a number of dwarf elliptical and irregular galaxies.) Like countless other galaxies, both past and future, our Galaxy may one day succumb to this `Virgocentric flow' and join its ranks. While the central region of the Virgo Cluster is dominated by a giant elliptical galaxy, the bulk of the cluster's visible mass is in the form of a hot (10±100-million-degree) gas that permeates the cluster. But Smith revealed the Virgo Cluster's dark secret. When he carried out the same calculations on the Virgo Cluster that Zwicky had performed on the Coma Cluster, he concluded that it contained an astonishing one hundred times more dark matter than luminous, and that this material probably lies between the galaxies. The virial theorem was a very direct argument for dark matter. Since Zwicky's work on the Coma Cluster, this same technique has been applied to many other clusters, yielding similar results. Far from shedding light on the nature of the Universe, astronomers were finding it to be an increasingly dark and mysterious place. In fact, astronomers are still using arguments not that much more sophisticated than Zwicky's original.
CONTRAST BETWEEN OORT AND ZWICKY Until the last 25 years or so, the dark matter argument presented by Oort seemed to have had more impact on most astronomers than that presented by Zwicky, even though the problem presented by Zwicky was much more extreme. Some people have speculated that this was due to the difference in the personalities of the two astronomers. The contrast between these two great men could not have been sharper. There was Oort ± the most pre-eminent Dutch (and probably world-wide) astronomer of his time, much beloved and revered by his fellow astronomers. Then there was Zwicky ± an irascible fellow who liked to disagree with the deeply held scientific beliefs of his fellow astronomers, no matter what