Unanswered Questions

The Structure and End of the Universe

Why do astronomers believe there is dark matter in the universe?

The first hints that there is more to the universe than visible stars and galaxies came in 1933 when the Swiss-American astronomer Fred Zwicky (1898-1974) studied the motion of galaxies in the Coma cluster and estimated that in order to account for their rapid motion there must be 400 times as much mass as could be accounted for. In the 1970s the American astronomer Vera Rubin (1938-) studied the rotation of galaxies and found that at least half of the matter in galaxies must be invisible—dark matter.

Dark matter is also in evidence in gravitational lensing, where the gravitational interaction of mass with light causes the light from very distant galaxies to be bent by nearer galaxies. The result is a distortion of shape and position in the images of the distant galaxies as if they are being seen through a lens. In one set of colliding galaxies the center of mass, determined by the lensing effect, is separated from the center of optical and X-ray brightness, probably by the effect of the collisions.

Radio and infrared emissions from the initial Big Bang that formed the universe is called the 3 kelvin cosmic microwave background radiation. Understanding the details of this radiation also requires large amounts of dark matter.


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