A stroboscope in a dark room picks out fleeting shadows from the crowd that the eye would otherwise miss. Similarly, the JWST telescope now takes frames every 21.5 seconds to catch short stellar flashes. If a tiny black hole (the size of a virus) passes in front of a star, its gravity will brighten the star's light for a few seconds, like a magnifying glass held over a lightbulb.
Normally, telescopes accumulate light over minutes, and rapid flashes are lost. Scientists built the FLASH program, which reads raw detector data and preserves each packet of light. This turns the telescope into a stroboscope. They checked 464,204 stars in the Andromeda galaxy but found no flashes.
So far, the stroboscope hasn't caught any shadows, but it has proven it can see them. If JWST is pointed at a single field and images it for hours without breaks, sensitivity will increase thousands of times. Then either the first flashes will appear, or this dark matter candidate will finally fade into the shadows.
At a mass of $10^{-9}$ solar masses, a primordial black hole has a Schwarzschild radius of about 3 nanometers, comparable to the size of a virus, yet its gravitational field can brighten a star in the Andromeda galaxy by a few percent.
The idea of invisible compact objects distorting the light of distant stars echoes the novel 'Roadside Picnic' by the Strugatsky brothers, where anomalous zones are left by mysterious aliens, although here the focus is on natural dark matter candidates.