Presented by Prof. Mike Hudson on 16th March 2018. Most of the matter in the Universe is dark matter: an elusive particle that is completely invisible. But we can “see” this matter by studying how it distorts the light from galaxies in the distant Universe, a phenomenon called gravitational lensing. I will give a whirlwind tour of gravitational lensing’s “greatest hits” showing how it can be used as a tool to understand some of the most mysterious things in the Universe: from black holes to the “cosmic web” of dark matter that links galaxies together.