A Moon-Mass Black Hole May Have Been Caught Drifting Through the Milky Way

A research team led by astrophysicist Renee Key of Swinburne University of Technology in Australia says it may have spotted a primordial black hole, a relic theorized to have formed within the first second after the Big Bang, briefly passing through the halo of the Milky Way.

The claim, described in two papers posted May 19, 2026, on the preprint server arXiv, centers on an object with roughly three times the mass of Earth's moon. Key and her colleagues say it was glimpsed as it drifted through the Milky Way's halo, the sparsely populated outer region of the galaxy believed to hold most of its dark matter.

Key herself acknowledges the result is contested, saying "the data have weaknesses." Still, the possibility of a discovery that could reshape understanding of the universe's history has drawn attention even among skeptics.

Primordial black holes were first proposed in the 1960s and studied in depth during the 1970s by physicists Bernard Carr and Stephen Hawking, who suggested that unusually dense pockets of matter in the young, expanding universe could have collapsed under their own gravity within fractions of a second of the Big Bang, producing black holes across a huge range of masses.

Because such objects would be extraordinarily hard to detect, they have long been considered a possible explanation for some or all of dark matter, the invisible substance that binds galaxies and galaxy clusters together. Over the decades, though, researchers have used increasingly refined methods to rule out many of the mass ranges in which primordial black holes could account for dark matter. "There is an enormous wealth of constraints on PBHs," said Djuna Croon, a theoretical particle physicist at Durham University who was not involved in the new work, calling a confirmed detection an "extraordinary" discovery.

Current estimates suggest dark matter particles could span masses from trillionths of an electron's mass up to roughly 1,000 times the mass of a proton. Tighter constraints on primordial black holes point to something closer to asteroid mass if they were to make up most of dark matter — making the newly proposed object, nicknamed "Phoebe" after the PBH abbreviation, something of an outlier.

A brief flash in the Magellanic Cloud

Key's team spotted the candidate using the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile, during five nights of observation in 2019. Each minute, the researchers photographed about ten million stars in the Large Magellanic Cloud, a dwarf galaxy roughly 163,000 light-years away, watching for stars that briefly brightened as a passing black hole's gravity magnified their light — a phenomenon called microlensing.

The team says it observed exactly that: a star estimated at about twice the size of the sun brightened sharply for roughly an hour before fading back to normal.

Such a flash could have other explanations, including natural stellar brightness fluctuations or a free-floating planet ejected from another star system, which can also produce a microlensing effect. But after extensive modeling, the team concluded the best explanation was a black hole three times the mass of the moon, passing through the galactic halo about 60,000 light-years from Earth at roughly 300 kilometers per second. Despite that mass, Key said the object would have a diameter "less than that of a human hair."

Because microlensing events depend on a one-time alignment, the object cannot be observed the same way again. The main way to test the hypothesis, said co-author Ken Freeman, an astronomer at the Australian National University, is to keep watching the star for renewed brightening — which "would be a strong indication that it has nothing to do with microlensing."

If primordial black holes exist, they might also help explain the puzzling early growth of supermassive black holes, including a 50-million-solar-mass object recently found by the James Webb Space Telescope just 700 million years after the Big Bang. "Maybe these supermassive black holes had a head start" from massive primordial black holes, suggested David Kaiser of the Massachusetts Institute of Technology.

Not everyone is convinced. Przemek Mróz, an astronomer at the University of Warsaw involved in the OGLE microlensing survey of the Magellanic Clouds, noted that moon-mass objects like Phoebe should have turned up in other surveys as well.