Back in 2018, scientists spotted a galaxy with stellar motions consistent with the galaxy not really having the invisible dark matter we thought was omnipresent in galaxies. Nicknamed DF2, this system is a giant blob of ancient stars. How you can get a galaxy without dark matter is not something scientists can answer. Heck, we thought galaxies formed in the large halos of dark matter that pulled in the luminous matter! But when researchers added up all the observed light from this system and measured all the motions, the observed stars were all that was needed to explain the observed motions.

Now, there is one variable that goes into these calculations that is pretty important: you have to know what distance to the galaxy so that you can calculate how big the stars actually are based on that distance and how bright they appear. In 2018, a distance of 65 million light-years was used based on a variety of distance calculations. If this estimate was just 30% off, and the galaxy was more like 42 million light-years away, then it would have a normal amount of dark matter and all would be right with science. 

To try and confirm the distance, a team led by Zili Shen re-examined this system and used forty orbits of the Hubble Space Telescope to get highly precise observations of individual stars. These measurements revealed with improved accuracy that DF-2 is not where scientists thought. It is actually farther away. In fact, the system is located 72 million light-years away. According to collaborator Pieter van Dokkum: Hubble really shows the entire thing. That’s it. It’s not just the tip of the iceberg, it’s the whole iceberg.

Somehow, there really is a galaxy with no dark matter. Theorists, it’s your turn.

More Information

NASA Goddard press release

IAS press release

“A Tip of the Red Giant Branch Distance of 22.1 ± 1.2 Mpc to the Dark Matter Deficient Galaxy NGC 1052–DF2 from 40 Orbits of Hubble Space Telescope Imaging,” Zili Shen et al., 2021 June 9, The Astrophysical Journal Letters