Gravitational Lensing as a Probe of Dark Matter
Sometimes we see MULTIPLE images of distant objects. The mass of the foreground object (galaxy here), bends the path of the light, creating false images:
We see incredible gravitational lensing in the direction of Clusters of Galaxies. This is seen in the image below. The amount of mass seen in visible matter is not enough to bend the light, meaning there is a lot of DARK MATTER associated with the cluster.
ALL THE BLUE ARCHES are from the exact same galaxy! How do we know? When we look at the light of each of the blue arches, they all have the EXACT SAME SPECTRUM.