Dark matter is an invisible window bending the light of galaxies. It was discovered by Vera Rubin, who noticed that stars were moving too fast. Perhaps this window is made of axions — particles billions of times lighter than an electron, invented by Roberto D. Peccei and Helen Quinn to fix the Standard Model. Axions are elusive: they can travel through a light-year of lead without a single collision. If dark matter is glass, axions are the scratches on it. Scientists searched for these scratches in the cosmic microwave background — the light of the Big Bang. Massive objects curve it, creating gravitational lenses. Three telescopes measured the distortions to figure out the fraction of axions. The result: for one axion mass, their share is no more than 1.5% of all dark matter. That’s like looking for a handful of sand in an Olympic swimming pool. For another type, a faint hint flickered — like a glint on glass — but it’s probably just a fluke. The window of the universe keeps its secrets.