§1Galaxies that spin too fast

In the solar system, the farther out a planet is, the more slowly it orbits. Neptune is far slower than Mercury. Because nearly all the mass is concentrated at the center (the Sun), orbital speed falls off as 1/√r with distance, just as Kepler's laws demand. In a spiral galaxy, too, most of the light is concentrated in the central bulge, so the outer stars ought to slow down accordingly.

Yet in the 1970s, the astronomer Vera Rubin measured the actual speeds of stars in the outer parts of galaxies, and found that far from slowing down, they kept a nearly constant speed even as the distance from the center grew. The visible stars alone cannot supply enough gravity to hold such fast-moving stars. Those stars should have been flung out of the galaxy — but they weren't. Something — an invisible mass — had to be wrapping the galaxy in a far wider envelope to explain the result.

§2Invisible, but it still has weight

Dark matter neither emits, absorbs, nor reflects light. It cannot be seen directly at any wavelength. But where there is mass there is gravity, and gravity cannot be hidden. The most dramatic evidence for its existence is gravitational lensing. A massive object bends the surrounding spacetime, so light coming from a distant galaxy behind it is bent as if passing through a lens on its way to us. By measuring how much the light bends, we can calculate how much mass lies in front of it — and the answer is a mass that the visible stars and gas come nowhere near accounting for.

The decisive scene is the Bullet Cluster of 2006. As two galaxy clusters collided, the hot gas (most of the visible ordinary matter) slammed together and lagged behind, while the center of mass — which sailed straight through the collision unaffected — ended up spatially offset from it. The "center of gravity" measured by lensing lay not with the gas but with something that had passed right through the other cluster — an observation that displays exactly the property of dark matter: it passes through almost without colliding.

§3Alternatives, and the cosmic inventory

There is also the alternative view: "What if we tweak the law of gravity itself, so that it can be explained without any invisible matter?" The leading example is MOND, which modifies Newtonian gravity at small accelerations and fits the rotation curves of individual galaxies quite well. But it struggles to account for phenomena like the Bullet Cluster's offset between mass and light, the fine structure of the cosmic microwave background, and observations on cluster scales all at once, so today the mainstream view is the dark matter hypothesis. That said, no experiment has yet directly detected a dark matter particle, so its identity remains an open question.

Put the many observations together and the makeup of the universe's matter and energy is astonishing. The ordinary matter made of atoms that we know is only about 5%, while dark matter is about 27%, and the dark energy that accelerates the expansion accounts for about 68%. The world that shines as stars and galaxies is just the tiniest tip of the cosmic iceberg.