via Smithsonian

The concept of dark matter originated in the 1930’s when astronomer Fritz Zwicky tracked the velocities of over 1,000 galaxies clustered together and observed that the gravitational pull from visible matter alone wasn’t strong enough to keep the cluster from flying apart. He posited that there must be matter that we can’t see—dark matter—that contributes most of the gravitational force that holds everything in place. Forty years later, astronomers Vera Rubin and Kent Ford found more evidence of dark matter by studying the motion of stars within spiral galaxies. They found that stars orbiting at the outer edges of these galaxies moved just as quickly as those at the center, possibly due to a halo of dark matter providing an extra gravitational pull. Most recently, a photograph of two colliding galaxies, nicknamed the Bullet Cluster, exhibited a gravitational lensing effect—light that is bent due to immense gravity—that could not be explained by visible matter alone. Scientists say that these observations strongly point to the existence of dark matter, but exactly what that matter is made of remains a mystery.

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