Gravitational lensing of the light from distant galaxies and quasars by closer galaxies or galaxy clusters enables us to calculate the amount of mass in the closer galaxy or galaxy cluster from the amount of bending of the light. The derived mass is greater than the amount of mass in the visible matter. Inventorying all of the ordinary matter in the lensing galaxy clusters (those that lens the light from distant galaxies) and comparing it to the total mass of the galaxy clusters gives a five-to-one ratio of dark matter to ordinary matter. An added feature of gravitational lensing is that it reveals dark matter even where there is no ordinary matter present so gravitational lensing can be used to map the distribution of dark matter within AND beyond the extent of the glowing, ordinary matter.