Dark Matter (Encyclopedia of Science)
Dark matter is the term astronomers use to describe material in the universe that does not emit or reflect light and is, therefore, invisible. Stars, nebulae, and galaxies are examples of luminous objects in the sky. However, luminous matter appears to make up only a small fraction of all the matter in the universe, perhaps only up to 10 percent. The rest of the matter is cold and dark, hidden from people's direct view.
The principal way dark matter can be detected is by observing its gravitational effect on nearby objects. Although dark matter does not shine, it still exerts a gravitational force on the matter around it. Astronomers believe that dark matter is a "cosmic glue" holding together rapidly spinning galaxies and controlling the rate at which the universe expands.
How can we know what we cannot see?
Understanding something that cannot be seen is difficult, but it is not impossible. Present-day astronomers study dark matter by its effects on the bright matter that can be observed. It was in the 1930s that Swiss astronomer Fritz Zwicky first pointed out that dark matter must exist. Zwicky claimed that the mass of known matter in galaxies is not great enough to generate the gravitational force to hold a cluster of galaxies together. Each independent galaxy moves at too great a speed for galaxies to remain in a cluster. Yet the galaxies...
(The entire section is 752 words.)
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