Normal lenses such as the ones used in a magnifying glass or a pair of spectacles bends the light rays that pass through them by refraction and focuses the light somewhere (such as in your eye). Gravitational lensing is among the first evidences of Albert Einstein’s general theory of relativity – simply put, mass bends light. The gravitational field of a massive object extends far into space; it bends the light rays passing close to that object and refocuses it somewhere else. The more massive the object, the stronger will be its gravitational field and hence greater will be the bending of light rays.
Astronomers are interested in large scale lensing that takes place in the universe. For instance, the gravitational field of galaxies and clusters of galaxies can lens light. However, in between the Earth and those galaxies is a mysterious entity called Dark matter. Though dark matter is invisible, it does have mass and makes up around 85% of the mass of the Universe. So, the light rays coming towards us from distant galaxies will pass through the gravitational field of dark matter and therefore will be bent by the lensing effect.