It seems that most of the universe is made up of mysterious ingredients which we cannot see directly. I will describe "gravitational lensing", the bending of light by gravity, which is predicted by Einstein's General Relativity. The dark components of the universe do not emit or absorb light, but do exert a gravitational attraction, and it turns out that gravitational lensing is one of the most promising methods for finding out more about them. This is very similar to looking through a bathroom window at streetlamps outside, and using the distorted images to learn about the patterns in the glass. I will describe the latest results from the Dark Energy Survey, which is observing shapes and approximate distances to 300 million galaxies over one eight of the entire sky.