Is this a moon-sized primordial black hole adrift in the Milky Way?

A flash of light that lasted only an hour, at the edge of the Milky Way, could be the trace of a black hole as old as the universe itself, with the mass of three moons and a diameter smaller than that of a human hair. This is not science fiction: it is the new frontier in the hunt for dark matter, and it has an almost affectionate nickname, Phoebe. For decades, we thought that dark matter was an exotic particle that had never been observed, something fundamentally new. But now a part of the scientific community is changing its mind: what if dark matter is instead composed of swarms of tiny primordial black holes, born in the first second after the Big Bang? The hypothesis was considered marginal, almost heretical, until attempts to find other answers failed. The theory is this: Phoebe could be the first concrete clue that primordial black holes really do exist and that, instead of being cosmic rarities, they are the real invisible "glue" that holds galaxies together. But who are the protagonists of this story? There is Renee Key, an Australian astrophysicist, who in 2019 led an observation campaign with the Dark Energy Camera in the Chilean Andes. Over five nights, she and her team photographed ten million stars every minute, hunting for a very rare variation in light: a microlensing event, that is, an invisible object that, passing in front of a star, briefly amplifies its brightness thanks to its gravity. And that's exactly where they found Phoebe. A striking detail: according to their calculations, that black hole travels at 300 kilometers per second and is located about 60,000 light-years away from us, in the peripheral area of the galaxy where most of the dark matter is thought to reside. But the scientific community is not united. Przemek Mróz, an astronomer from Warsaw, points out that if Phoebe were really a lunar-mass black hole, we should have already observed hundreds of them in other similar experiments like OGLE. Yet, he says, we don't see them, so maybe it's just an ordinary variable star. There are also those, like David Kaiser of MIT, who go further: if primordial black holes exist, they could explain why supermassive black holes appear so early in the history of the universe, such as the one with 50 million solar masses observed by the James Webb Telescope only 700 million years after the Big Bang. Another way: to look for primordial black holes through their final explosions. According to Stephen Hawking's theory, black holes slowly evaporate, emitting radiation, and eventually explode in a flash of energy. One group has hypothesized that a very energetic neutrino detected off the coast of Sicily could be the signal of a primordial black hole that has just dissolved, but others remain skeptical: if this were really the case, we should also see gamma rays, which are missing. One perspective that no one is considering enough is the psychological burden of this hunt: dark matter is the greatest obsession of modern cosmology, and every new alternative hypothesis immediately becomes a battleground between those who want to change the paradigm and those who prefer to remain anchored to the old ways of thinking. In the end, the sentence to remember is this: dark matter may not be something new, but something very ancient that we have always had right under our noses — black holes born when the universe was less than a second old. If this story interests you, you can press I'm In on Lara Notes — it's not a like, it's your way of saying: this idea is now mine. And if tomorrow you tell someone that dark matter could be made of primordial black holes called Phoebe, on Lara Notes you can mark it with Shared Offline — because the best conversations deserve to be remembered. This was Scientific American, and so you saved 4 minutes of reading.