Most galaxies are dominated by dark matter. Our own galaxy is about 95% dark matter by mass. But recently a galaxy was discovered that is almost entirely dark matter. Known as Dragonfly 44, less than 0.01% of its mass is dark matter. Dark matter is highly mysterious due to the fact that we haven’t detected it directly. Since dark matter doesn’t interact with light, we can only observe its gravitational effect on light and the motions of stars. The elusive nature of dark matter has led some to propose other ideas such as modified gravity, but these don’t agree well with the Universe we observe. Given that dark matter is so mysterious, how do we know Dragonfly 44 is a dark matter galaxy?