The full galaxy cluster acts as a gravitational lense on galaxies behind.

These become largely distorted and they are visible in this picture partly as redish and partly bluish arc-like structures. From the position and the distortion of many such luminous arcs it is possible to determine the complete mass and the distribution of the cluster.

One finds, that the bright stars in the galaxy cluster only provide one tenth to the complete mass, the rest is due to Dark Matter.

Gravitational lensing offers a way to determine the contribution of Dark Matter independent of other astrophysical methods.