How to Spot a Galaxy Made of Dark Matter

A galaxy made of 99.94% to 99.98% dark matter immediately challenges and updates models for how dark matter works. It also sheds light on how the universe arrived at the large-scale distribution of matter we see.

The galaxy in question is called CDG-2, or Candidate Dark Galaxy 2. But how did astronomers manage to find something that's basically defined by how difficult it is to locate?

For this international team of researchers, the first step was looking for an unusual arrangement of globular clusters. These objects are very high-density collections of thousands of old stars, thought to be relics of the earliest stages of star formation in primordial regions of the universe. Globular clusters are generally found around large amounts of matter, in particular around the exteriors of galaxies.

Seen from far enough away, these globular clusters would seem to trace out the exterior of a galaxy ...