There are also two large blue-colored regions in the composite image. these blue regions are labeled as "dark matter" based on

There are also two large blue-colored regions in the composite image. These blue regions are labeled as "dark matter" based on observations of gravitational lensing by the cluster.

A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer.

f=gm1m2/d^2 it d

f is force g is gravitaiton constant, m1 is mass of first object, m2 is mass of second object. d is distance between two object