In 1905 Einstein published four papers that changed our understanding of space-time and laid the ground for the quantum theory of light. In the following decade, he developed his theory of gravity known as General Relativity (GR). Nevertheless, these achievements were not well known outside Germany. This changed overnight when the British astronomer Arthur Eddington used observations of a total solar eclipse in 1919 to demonstrate that background light is bent as it passes the Sun. This confirmation of General Relativity made Einstein world-famous. In this talk, we will describe the physical origin of light deflection and discuss how this phenomenon is currently being used to map the distribution of dark matter in the Universe.