Glossary

Dark Matter

Dark matter is an invisible form of matter that makes up about 27 percent of the universe, detected only through its gravity. Galaxies spin too fast for the visible matter they contain to hold them together, and galaxy clusters bend light more strongly than their stars and gas can explain. The Bullet Cluster, where the gravitational mass separated cleanly from the ordinary gas during a collision, is among the strongest evidence that dark matter is real and not merely a flaw in our theory of gravity.

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