An Overview of the Theories on Dark Matter

History

It was always known that there was matter in the sky that we couldn't see. Dark matter makes up 90% of the universe. It cannot emit, absorb or reflect light, which makes it truly black. It is also easy to confuse dark matter with antimatter. If dark matter is made out of antimatter, then all matter in our universe would have already been annihilated. It is pretty safe to assume that dark matter is not made out of antimatter (5, 6). The first evidence that there was a significant amount of matter that we couldn't see was from examining clusters of galaxies. In 1933, the astronomer Fritz Zwicky was studying a couple clusters of distant galaxies. He estimated the total mass of a group of galaxies by measuring their brightness.

When he used a different method to calculate the mass again, he came up with a number that was 400 times his original estimation. This inconsistency and difference in masses is now known as "the missing mass problem." Though, that is a misleading term, since the mass is there, but rather it is light that is missing (2). Zwicky said, “Matter, although invisible, can still be demonstrated through its gravitational clout.” He was the first to suggest that invisible matter could be detected indirectly by its gravitational effects (7). He was the first to pointed out in the 1930s that dark matter must exist. The reason for this, he claimed, was that the mass of known matter in galaxies is not great enough to hold a cluster of galaxies together. Each independent galaxy moves at too great a speed for the galaxies to remain in a cluster. Yet the galaxies were not spinning away from each other. They had to be held together by a gravitational field created by undetected mass (8).