In the vast expanse of space, not everything we observe tells the full story. For centuries, astronomers believed that the stars, planets, gas clouds, and galaxies we could see were all there was to the universe. However, observations in the twentieth century began to suggest otherwise. The cosmos, as it turns out, is filled with something we cannot directly observesomething invisible yet undeniably present. This hidden substance came to be known as dark matter, and its discovery revolutionized our understanding of the universe. The first real hints of dark matter came from studies of galaxies. When scientists examined how stars move within galaxies, they expected the outer stars to move more slowly than those near the center, based on the distribution of visible mass. However, what they found was shocking. Stars far from the center were moving at similar speeds as those near the core. This was only possible if there was a large amount of unseen mass exerting gravitational force throughout the galaxy. This discrepancy between the predicted and observed rotation speeds became one of the earliest and strongest pieces of evidence for the existence of dark matter. At the same time, astronomers began noticing unusual behavior in galaxy clusters. These massive structures, made up of hundreds or even thousands of galaxies bound together by gravity, seemed to have more mass than what could be accounted for by visible matter alone. The galaxies were moving too fast to be held together by the gravity of the stars and gas we could detect. Something else, something invisible, had to be holding them in place. Again, the idea of an unseen mass emerged, one that made up the majority of the clusters mass.
