A few years ago, astronomers discovered Dragonfly 44, a galaxy seemingly dominated by dark matter. This finding challenged our models of galaxy formation. Now, a new study from leading experts resolves the puzzle.
The concept of dark matter emerged in the 1970s through pioneering work by Vera Rubin. She observed that stars in the Milky Way orbit at speeds requiring far more mass than visible gas, dust, and stars could provide.
This led to the hypothesis of an invisible form of matter holding galaxies together. Today, dark matter is estimated to comprise 26.8% of the universe, compared to just 4.9% for ordinary matter—the rest being dark energy.
Typically, dark matter around galaxies ranges from 10 to 300 times the mass of visible material. But Dragonfly 44, located in the Coma cluster, appeared anomalous.
Initial observations suggested Dragonfly 44 harbored dark matter equivalent to a billion solar masses—like the Milky Way—yet it contained only about 100 million stars, roughly a thousandth as many.
This implied dark matter outweighed stars by 10,000 times, defying expectations and prompting intense scrutiny.
Astronomers at the Kapteyn Astronomical Institute, University of Groningen, Netherlands, re-examined Dragonfly 44 by studying its globular clusters, whose numbers correlate with a galaxy's total mass.
They found the cluster count had been overestimated: only about 20, not 80 as previously reported. Adjusting for this, dark matter is now just 300 times the luminous matter—in line with similar ultra-diffuse galaxies.
"With this revised globular cluster count, Dragonfly 44's dark matter aligns perfectly with expectations for its type," says lead author Ignacio Trujillo. "The visible-to-dark matter ratio drops from 1:10,000 to 1:300."
Co-author Teymoor Saifollahi adds, "Dragonfly 44 was an unexplained outlier. Now we see the earlier results were off—it's business as usual for galaxy models."
The findings are detailed in Monthly Notices of the Royal Astronomical Society (MNRAS).
