Segue 1 is a small satellite galaxy orbiting the Milky Way. It has few visible stars, but is very dense with dark matter, so that its gravity is approximately 1,000 times as strong as its luminosity would suggest. Segue 1 is among the most extreme examples of a dark matter galaxy known.
The galaxy is one of many nearly invisible galaxies discovered by the Sloan Digital Sky Survey. These galaxies have a ratio of dark matter to regular, visible matter of 100 or 1,000 to one, making them difficult to detect. In contrast, the Milky Way galaxy has a ratio of approximately 10 to one.