Mysterious 2019 stellar brightening could reveal dark matter's secrets

On the morning of December 18, 2019, a star in the Large Magellanic Cloud did something unusual. Its light grew brighter, then dimmed again, all within the span of a single hour. The change was gentle, symmetrical—not the violent flare of an explosion, but a smooth swell and recession of luminosity. Telescopes at the Víctor M. Blanco Observatory in Chile, perched high in the Andes, caught the event. The scientists studying the data gave the mysterious object a name: Phoebe.

For years, Phoebe remained an enigma. What could cause such a precise, fleeting brightening? The answer, researchers now believe, points toward something far older than any star—possibly one of the universe's most ancient objects. A team from Swinburne University has published calculations suggesting that what passed in front of that distant star was a primordial black hole, a theoretical relic from the first seconds after the Big Bang itself.

The mechanism behind the brightening is well understood: gravitational microlensing. When a massive object drifts between Earth and a distant star, its gravity warps spacetime like a lens, magnifying the star's light. The brighter the magnification and the shorter the duration, the lighter the lensing object must be. Phoebe's one-hour event was remarkably brief in cosmic terms—right at the edge of detectability. This timing revealed something crucial: the object causing the effect weighed roughly three lunar masses. That is extraordinarily light for an astronomical body.

Three lunar masses eliminates most conventional explanations. Stellar black holes, born from collapsed stars, typically weigh at least five times the sun. A rogue planet wandering through the galaxy would be too massive. The geometry of the event and its spatial distribution pointed instead toward something else entirely: a primordial black hole. These are not born from stellar death. Instead, they would have formed in the universe's first moments, when density fluctuations in the primordial cosmos created pockets dense enough to collapse into black holes before any stars existed.

The probability calculations strongly favor this explanation—by a factor of 100,000 over competing theories. Yet primordial black holes remain theoretical. No one has definitively detected one. If Phoebe is indeed what the Swinburne team believes, it would represent something far larger than the discovery of a single object. It would be evidence for an answer to one of astronomy's deepest questions: what is dark matter?

The visible universe—all the stars, planets, gas, and dust we can see—makes up only five percent of existence. The rest is unknown. Part of it is dark energy, which drives cosmic expansion. The rest is dark matter, an invisible substance whose gravitational influence shapes galaxies and galaxy clusters. No one knows what dark matter is made of. One leading hypothesis suggests that primordial black holes could account for at least some of it. If Phoebe is confirmed as primordial, it would be the first concrete evidence that this ancient population of black holes actually exists and contributes to the dark matter that dominates the cosmos.

The work is far from finished. Scientists must search for more events like Phoebe's to build a convincing case. This requires telescopes sensitive enough to detect subtle brightness changes in distant stars, capable of scanning wide fields of view, and ideally focused on regions crowded with stars—places where gravitational lensing events are more likely to occur. The Vera Rubin Observatory, coming online soon, is expected to generate vast datasets suitable for this hunt. Researchers will comb through the data looking for signatures matching Phoebe's pattern.

On that December morning in 2019, while a pandemic was beginning to grip the world below, something ancient may have drifted past a distant star. If the scientists are right, that brief brightening could unlock one of the universe's greatest secrets—what the cosmos is actually made of.