This image shows the galaxy MACS J0416.1–2403, one of six clusters targeted by the Hubble Frontier Fields programme.
The varying intensity of blue haze in this image is a mass map created by using new Hubble observations combined with the magnifying power of a process known as gravitational lensing.
Strong lensing gives a much more precise indication of the mass at the cluster’s core whilst weak lensing provides valuable information about the mass surrounding the cluster core.
Image Credit: ESA/Hubble, NASA, HST Frontier Fields
Explanation from: https://www.spacetelescope.org/images/heic1416c/