Dark matter is the mysterious nonluminous material that makes up about most of the universe.  Dark energy is a mysterious repulsive force. Together they make up about 95% of the universe. The rest--all observable matter--adds up to less than 5% of the universe.  Nearby dwarf galaxies have the highest measured densities of dark matter, making them ideal for dark matter studies, but that proximity also has a downside. Star systems so close to the massive Milky Way are subject to the acceleration of their stars by our galaxy’s tidal forces, an effect that can mimic the presence of dark matter. The lack of bright stars in dim dwarfs also makes it difficult to measure the velocities of enough stars for sufficient certainty.

Astronomer Josh Simon and colleagues have determined that a tiny, very dim galaxy orbiting the Milky Way, called Segue 1, is the darkest galaxy ever found and has the highest dark matter density ever found. His team has also laid to rest a debate about whether Segue 1 really is a galaxy or a globular cluster—a smaller group of stars that lacks dark matter. Their findings make Segue 1 a promising laboratory to study dark matter, particularly the possibility that dark matter could be seen for the first time via a detection of gamma rays emanating from colliding dark matter particles.

Simon and company measured and analyzed the speed and chemistry of 397 stars in the vicinity of Segue 1. He and collaborators ruled out other possible causes of the dark-matter behavior, suggesting  that Segue 1 and other dwarfs could  provide astronomers with concrete proof that their dark matter theories are on the right track. Image courtesy NASA