Dark matter is an invisible substance that reveals itself only through gravitational effects: it bends the light of distant galaxies, much like an unseen window that alters the path of light rays. Its existence was first convincingly demonstrated by Vera Rubin, who measured the rotation speeds of stars in galaxies and found that they move too fast, as if held together by additional, invisible mass. One hypothesis suggests that this "window" might be made of axions — hypothetical particles billions of times lighter than an electron. They were originally proposed by Roberto D. Peccei and Helen Quinn to solve a theoretical problem in the Standard Model of particle physics. Axions interact extremely weakly with ordinary matter: they can travel a light-year through lead without a single collision. To extend the analogy, dark matter is the glass, and axions are microscopic scratches on it — almost invisible, yet capable of affecting the light that passes through.

To estimate how many such "scratches" might be hidden in the Universe, scientists turned to the cosmic microwave background — the ancient light left over from the Big Bang. Massive objects bend its path, creating the effect of gravitational lensing — distortions similar to those seen when looking through uneven glass. Using three telescopes, researchers measured these distortions to calculate what fraction of dark matter could consist of axions.

The result was a stringent constraint: for one possible axion mass, their contribution does not exceed 1.5% of all dark matter. The scale of this smallness can be compared to finding a handful of sand dissolved in an Olympic-sized swimming pool. For another type of axion, the data showed a faint hint, like a glint on glass that might indicate their presence, but most likely it is a statistical fluke. The cosmic window continues to keep its secrets, and axions remain just one possible, but as yet unconfirmed, solution.