The universe is permeated by invisible mass, dark matter, whose existence was first proposed by Fritz Zwicky in the 1930s, based on his observations of galaxies moving in clusters. Today, one of the most intriguing candidates for this role is the axion, a hypothetical particle beyond the Standard Model of particle physics. To hunt for them, researchers are adapting laser interferometers — ultrasensitive instruments originally built to detect gravitational waves, ripples in spacetime. Here's how they work: a laser beam is split into two perpendicular beams that travel along kilometer-scale arms and then recombine; any tiny change in path length caused by a gravitational wave alters the interference pattern. Similarly, when an axion flies through the laser beam, it might slightly change the polarization or phase of the light, like a faint whisper layered over the main tone. In this way, the interferometer becomes an 'ear' capable of picking up the nearly intangible influence of dark matter.

The ground-based detector LIGO, with arms a few kilometers long, is not yet sensitive enough for such a search. However, the future space observatory LISA, which will stretch beams across millions of kilometers in space, could, over a year of observations, pick up a signal a thousand times fainter than what current instruments can detect. Moreover, if dark matter forms clumps, as expected in some models, their passage through the beam would imprint a distinctive temporal rhythm on the signal, helping to filter out random noise. Interestingly, the name 'axion' was borrowed from a brand of laundry detergent: the particle was meant to 'clean up' the theoretical mess in quantum chromodynamics, resolving the so-called strong CP problem.