Why Galaxies Spin the Wrong Way: The Dark Matter Mystery

In the 1970s, astronomer Vera Rubin pointed a telescope at the Andromeda Galaxy and measured how fast its stars were moving. Newton's laws of gravity predict that stars near the outer edge of a spinning galaxy should move much slower than stars close to the center, the same way outer planets in our solar system orbit the sun more slowly than Mercury does. But that is not what Rubin found. Stars at the edge of Andromeda were zooming around just as fast as stars near the middle. Something invisible had to be pulling on them. That something is called dark matter, and it makes up far more of the universe than anything we can actually see.

- Why Vera Rubin's star measurements broke the rules of gravity as we knew them - What dark matter is and why scientists are sure it exists even though no telescope has ever photographed it - How dark energy is different from dark matter, and why it's stretching the universe apart faster every year - How astronomers use gravitational lensing to weigh invisible matter

A galaxy's rotation curve is a graph of how fast stars orbit at different distances from the center. Based on the visible stars, gas, and dust alone, that curve should dip downward at the edges, with outer stars moving slower, just like Neptune orbits the sun much more slowly than Earth does. Instead, when Rubin and her colleague Kent Ford plotted more than 60 spiral galaxies through the 1970s and 1980s, almost every rotation curve came out flat. Stars at the outer edge moved just as fast as stars near the core. For the math to work, each galaxy needed roughly five to six times more mass than all of its stars, gas, and dust combined.

Dark matter does not glow, reflect light, or block light the way a planet or a cloud of dust does, which is exactly why it took so long to notice it was there. Scientists know it exists because of its gravity, not because they've ever seen it directly. The best current estimate, based on data from the Planck satellite, is that dark matter makes up about 27% of everything in the universe. Ordinary matter, every star, planet, tree, and person, is only about 5%. One of the strongest pieces of evidence comes from the Bullet Cluster, a pair of colliding galaxy clusters 3.7 billion light-years away, where astronomers could see that most of the mass had separated from the visible gas during the collision.

Dark energy is a separate mystery from dark matter. In 1998, two teams of astronomers studying distant exploding stars called Type Ia supernovae discovered that the universe isn't just expanding, the expansion is speeding up. Something is pushing galaxies apart faster and faster, and scientists named that unknown force dark energy. Dark energy makes up about 68% of the universe, which means the matter we can see, stars, planets, you, is a small sliver of everything that exists.