Stars in the Dark

For centuries, humans have gazed into the night sky, believing that what they saw—stars, planets, glowing nebulae—was all there was. But as our tools for exploring the universe improved, scientists began to notice something strange. The visible matter in galaxies simply wasn’t enough to explain how those galaxies moved and behaved. Something unseen, something invisible, seemed to be exerting a powerful influence on everything. In the 1930s, astronomer Fritz Zwicky was studying a group of galaxies known as the Coma Cluster. When he measured how fast the galaxies within the cluster were moving, he realized they were traveling far too quickly to be held together by the visible matter alone. According to the laws of gravity, the cluster should have torn itself apart. Zwicky proposed that some kind of “missing mass” must be present, something that added gravitational force without emitting light. He called it “dark matter.” At the time, his idea was mostly ignored. Decades later, in the 1970s, American astronomer Vera Rubin provided even more compelling evidence. She was studying the rotation of spiral galaxies, expecting that stars farther from the center would orbit more slowly, just as planets farther from the sun move more slowly in our solar system. But that wasn’t what she found. The outer stars were moving just as fast as those closer in, a finding that didn’t make sense if only visible matter was involved. Her work suggested that some unseen material—something with mass—was surrounding the galaxies in large halos, exerting gravitational pull and affecting their motion. This was dark matter at work.