A Galactic Mystery

Dwarf galaxies without dark matter present a cosmic conundrum

- BY MARIA LUÍSA BUZZO ILLUSTRATION BY CHAD HAGEN

THE FIRST TIME I HEARD ABOUT GALAXIES without dark matter, I was sitting in my very first graduate class at the University of São Paulo. It was 2018, and the discovery had just been announced. A team had found a small, strange galaxy that appeared to lack dark matter, the invisible material that was thought to make up most of the matter in the universe and considered essential to the formation, evolution and stability of galaxies. The find was big enough that it made it to Brazilian television.

Our professor used the news to open the semester. It sparked months of discussions. Dwarf galaxies—smaller, puffier conglomerations of stars than spiral galaxies such as the Milky Way—were long thought to be dominated by dark matter. Could they actually form and survive without it? Was the result real, or could it reflect flawed assumptions or uncertainties? Were we witnessing a problem in our models of galaxy formation?

Again and again we returned to a deceptively simple but surprisingly hard question: What defines a galaxy? It turns out there’s no single answer. For example, in 2011 two astronomers, Duncan Forbes of Swinburne University in Australia and Pavel Kroupa of the University of Bonn in Germany, conducted a survey called What Is a Galaxy? Their results highlighted just how varied the definition can be, even among experts. Most astronomers agree on these basics: galaxies are massive, gravitationally bound systems of stars, gas and dust, with an important but poorly understood complement of dark matter. The provocative discovery of a galaxy that apparently lacked this invisible stuff called that definition into question.