Two people have been credited for determining the need for dark matter. The first was astronomer Jan Oort, who back in 1932 measured the perpendicular motions of nearby stars relative to the disk of our galaxy. By studying the gravitational influence of the disk on these stars he was able to calculate the mass of the disk. The value he determined was twice as much as that inferred by the visible stars and nebulae in that region. A year later in 1933 astronomer Fritz Zwicky estimated the mass of a group of galaxies by measuring their brightness. When Zwicky used a different method of calculating the mass of the cluster he found it to be 400 times the value from directly inferring the mass from the brightness of the cluster. In both cases Oort and Zxwicky came to the conclusion that the visible galaxies only accounted for 10% of the mass needed to keep them gravitationally in the cluster.
Based on 70 years of accumulated observations of the motions of galaxies and the expansion of the universe, most astronomers believe that as much as 90% of the stuff constituting
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