Gravitational lensing by galaxies, Hubble space telescope image. The red colour of the middle galaxy indicates that it is older. Just above it is a blue arc. This is caused by light from a galaxy further away being bent and distorted by the mass of the red galaxy. Einstein's theory of relativity predicted that such an effect would occur and would be proportional to the mass of the distorting object. This phenomenon can therefore be used to calculate the mass of the red galaxy.