Although we live in a renaissance era of cosmology, in which theories and observations have advanced to the stage where ideas can be precisely tested, we also live in the dark ages. About 23 percent of the universe consists of dark matter, mysterious stuff that exerts gravitational forces but doesn’t interact with light. Ordinary matter makes up just 4 percent. (Another 73 percent is dark energy, an even more mysterious component that permeates the universe.)
The last time something was called “dark” in physics was in the mid-1800s, when Urbain-Jean-Joseph Leverrier of France proposed an unseen dark planet, which he named Vulcan. Leverrier’s goal was to explain the peculiar trajectory of the planet Mercury. Leverrier, along with John C. Adams of England, had previously deduced the existence of Neptune based on its effects on the planet Uranus. Yet he was wrong about Mercury. It turned out that the reason for ...
