There’s something strange going on with the Milky Way. Recent measurements suggest that stars at the outskirts of our galaxy are misbehaving. They’re traveling far slower than similarly situated stars in other galaxies. One possible explanation for the Milky Way’s stellar slowpokes is that our galaxy is extraordinarily deficient in dark matter, the invisible substance thought to serve as gravitational scaffolding for cosmic structures. Another is that our core conceptions about dark matter—such as how much of it exists in the universe—are somehow deeply flawed.