The light left over from the Big Bang — the cosmic microwave background — is like a giant ribbon threading through the cosmos. It oscillates in a single plane, but astronomers noticed that this plane is slightly twisted over billions of years. The previous explanation required hypothetical particles axions, going beyond the Standard Model, but they haven't been found. A new approach shows: the twist arises naturally when the ribbon of light crosses invisible boundaries in dark matter. Each crossing gives a tiny rotation — a purely geometric effect, independent of the ribbon's color. Whether radio waves or gamma rays, the twist is the same. This effect is known as a geometric phase and needs no new particles. Such a mechanism allows us to study the hidden structure of the universe. Future polarization measurements of the sky will test the idea, while computer models will link it to the expansion of the universe. As Wheeler foresaw, the geometry of space itself governs light. The works of Maxwell and Lorentz on electromagnetism laid the foundations, and now perhaps string theory will find echoes here.