In December 2019, astronomers detected a one hour brightening of a star in the Large Magellanic Cloud, a classic gravitational microlensing event. These occur when a compact object bends a distant the light of a distant star as it passes in front of it. The object responsible in this instance, named Phoebe, has a mass of roughly three times that of our Moon making it far too small to be a stellar black hole, but consistent with a primordial black hole formed moments after the Big Bang.

…That something has been named Phoebe. And working out what it actually is turns out to be one of the most intriguing puzzles in modern astronomy. The phenomenon at the heart of the story is called gravitational microlensing and it’s one of the most elegant predictions of Einstein’s general theory of relativity. When a massive compact object passes between us and a distant star, its gravity acts like a lens, briefly magnifying the star’s light in a very characteristic way. The shape of the brightening is distinctive and entirely unlike anything produced by a variable star, a flare, or an asteroid…

The team calculated the probability of the lensing object belonging to each possible population — Milky Way stars, Large Magellanic Cloud stars, or the dark matter halo between and around them. The dark matter halo wins by a factor of 100,000. Phoebe is five orders of magnitude more likely to be a dark matter object than anything associated with normal stellar matter…