Two invisible components, dark energy and dark matter, make up most of the energy content of the Universe and yet we know very little about either. As far as we know, dark matter cannot be observed directly. In order to study its distribution in the Universe, we use a technique called gravitational lensing. As predicted by Einstein’s general relativity, mass curves spacetime. Consequently, light rays are differentially deflected in a gravitational potential. If there is a structure along the line of sight between an observer and a distant object, a perturbation in the photon trajectory is caused. As a consequence, the position of the image of the source will be slightly shifted and magnified, and, if extended, it will also be distorted.