How to Tell if Dark Matter Is Not Perfectly Smooth

If there is a big clump of dark matter, it will affect how stars are moving around it. It will change the motion of those stars and attract or deflect them. That's hard to tell because this kind of gravitational effect is kind of rare. We recently launched a satellite called gaya which is mapping the galaxy in four dimensions. So we're getting this incredible map with like a billion stars with their position and their velocity mapped. And we can use this to look for deviations - if dark metter was perfectly smooth, what would we expect all these stars to be doing? You'd expect things at the same radeus to basically having the same motion. And so you see deviations