Gravitational Lensing

One of the most intriguing predictions of Einstein's theory of gravity, General Relativity, is that light rays are bent by massive objects.  For example, if two galaxies are closely lined up from our point of view, the foreground galaxy can magnify the image of the background galaxy or create multiple images, etc.   The source for the gravity of the foreground object could even be the mysterious substance "Dark matter".  Hence gravitational lensing is a tool that can tell us something about the distribution of Dark matter - see the preprint Gravitational Lensing