For nearly a century, astronomers have been searching for dark matter — an invisible scaffold that keeps galaxies from flying apart. Fritz Zwicky first proposed it, and Vera Rubin proved its existence by showing that stars on the outskirts of galaxies move too fast. Stephen Hawking suggested that dark matter could be made of primordial black holes, born in the fire of the Big Bang.

Recent observations of the galaxy Andromeda with the Subaru telescope seemed to confirm the hypothesis: ultra-precise photometry (measuring brightness) of millions of stars revealed 12 brief flashes. They were explained by gravitational microlensing: a passing black hole, like a magnifying glass, momentarily boosted the light of a background star.

But a reanalysis of the data noticed that the flashes were asymmetrical and sometimes repeated. It turned out these weren't lenses, but beacon stars themselves, whose brightness changes rhythmically. Ten objects are RR Lyrae type: they pulsate with a period of less than a day, like a distant lighthouse. Another is an eclipsing pair, where two stars take turns blocking each other's light. The last remains a mystery. There are no black holes in dark matter. This case reminds us: even the most elegant theories need rigorous checking — it's easy to mistake the twinkling of a lighthouse for a signal from the abyss.