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==Evidence for dark matter==
In 1913, Norwegian explorer and physicist [[Kristian Birkeland]] may have been the first to predict that space is not only a [[plasma]], but also contains "dark matter". He wrote: ''"It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in 'empty' space".'' (Ref. See notes)Professor Peebles and Professor Ostriker, both of [[
Dark matter was first hypothesized to exist by the [[Switzerland|Swiss]] [[astrophysicist]] [[Fritz Zwicky]]. In 1933 Zwicky estimated the total amount of mass in a [[galaxy cluster|cluster of galaxies]], the [[Coma Cluster]], based on the motions of the galaxies near the edge of the cluster. When he compared this mass estimate to one based on the number of galaxies and total brightness of the cluster, he found that there was about 400 times more mass than expected. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the "missing mass problem". Based on these conclusions, Zwicky inferred that there must be some other form of matter existent in the cluster which we have not detected, which provides enough of the mass and gravity to hold the cluster together.
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