axion

Sometimes a new particle is proposed not to explain dark matter, but to fix an unrelated puzzle — and only later turns out to be a perfect dark-matter candidate. The axion is exactly this kind of lucky accident. It was dreamed up in the 1970s to solve a subtle problem in the theory of the strong nuclear force (why that force does not break a symmetry called CP, when it easily could). The fix required a brand-new, extremely lightweight particle, which was christened the axion — reportedly after a brand of laundry detergent, because it 'cleaned up' the problem.

The axion would be astonishingly light — perhaps a trillion times lighter than an electron — and would barely interact with anything, which is why it has never been seen. But if axions were produced in the early universe, vast numbers of them would still be drifting through space today, moving slowly and clumping under gravity. In other words, an ultra-light axion behaves exactly like cold dark matter, even though, particle by particle, it weighs almost nothing. A sea of these featherweight particles could quietly add up to all the missing mass.

Searching for axions calls for very different tools than hunting WIMPs. Because an axion can, in a strong magnetic field, occasionally convert into a faint radio-wave photon, experiments use sensitive tuned cavities inside powerful magnets, slowly scanning across possible axion masses like turning a radio dial in search of a faint station. With WIMP searches coming up empty, the axion has become one of the most actively pursued dark-matter candidates, though it too remains undetected so far.