On 18 December 2019, a single star in the Large Magellanic Cloud, our galaxy's small companion, grew brighter for roughly an hour and then faded back to normal. The rise and fall were smooth and symmetric, unlike any flare or ordinary stellar variation, and the star never flickered again.
A team led by Renee Key at Swinburne University of Technology in Melbourne believes it witnessed gravitational microlensing: a compact object drifting across the line of sight, its gravity bending and briefly magnifying the star's light exactly as Einstein's general relativity predicts. They have named the unseen lens Phoebe, a nod to the abbreviation PBH — primordial black hole.
The observation came from the AMPM survey (Asteroid-Mass Primordial black hole Microlensing), which used the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile over five nights, monitoring about ten million stars in the Large Magellanic Cloud every minute. The two resulting papers appeared on the arXiv preprint server on 19 May 2026.
What Phoebe might be
Because the mass of a lens sets how quickly it crosses in front of a star, the event's duration is the key clue. At about 60 minutes it is among the shortest and lowest-mass microlensing signals ever recorded. Bayesian modelling points to a mass of roughly three lunar masses — far too little for a wandering planet, and far below the roughly five solar masses that mark the smallest black holes born from collapsing stars.
The only object that light and that dense would be a black hole forged in the first fractions of a second after the Big Bang. A probabilistic analysis found Phoebe five orders of magnitude more likely to belong to the Milky Way's dark-matter halo than to any ordinary stellar population; the team places it some 60,000 light-years away, sweeping through the halo at about 300 kilometres per second, yet smaller across than a human hair.
The claim is contested, and Key readily notes that "the data have weaknesses." Alternative explanations — a natural stellar hiccup, or a free-floating planet ejected from a distant system — remain on the table. Even so, the result shows how microlensing can be turned into a precise dark-matter probe, and independent physicist Djuna Croon of Durham University calls a confirmed detection an "extraordinary" prospect worth chasing.