In the dark side of the
universe: the axion hypothesis

It does not emit or absorb electromagnetic radiation, i.e. it cannot be detected by our telescopes; it interacts very little with the atoms of ordinary matter; it is so exotic that it has no place on the periodic table of elements: it is dark matter, a very common form of matter, accounting for 85% of the total mass of the universe, yet invisible, detectable only indirectly, through its gravitational effects. Although the first hypothesis of its existence dates back to 1933, it only began to gain traction in the 1970s, when American astronomers Vera Rubin and Kent Ford measured the rotational speed of stars on the outskirts of the Andromeda galaxy. They found a value that was surprisingly high compared to Newton’s predictions of gravity, and the most plausible explanation for this anomaly was to assume the existence of a form of invisible matter, called “dark matter”, whose contribution (added to that of ordinary matter) would explain the measured speed. Other indirect evidence followed, and a proliferation of theoretical models attempting to understand and justify its existence: among these, one predicts that dark matter is composed of axions, particles whose detection Belgian physicist Pierre Sikivie has dedicated decades of work throughout his scientific career. In February, Sikivie was awarded the INFN Galileo Galilei Medal 2025 for his “pioneering contributions to our understanding of the cosmos, in the quest to make the ‘invisible axion’ visible”, and in this month, which marks Dark Matter Day on October 31, we retraced with him his efforts to uncover the nature of dark matter.