Galaxy cluster Abell 520's mass (HST WFPC2)

This image shows the location of most of the mass in merging galaxy cluster Abell 520's core, which is dominated by dark matter. Dark matter is an invisible substance that makes up most of the Universe's mass. The dark-matter map was derived from Hubble Wide Field Planetary Camera 2 observations, by detecting how light from distant objects is distorted by the cluster galaxies, an effect called gravitational lensing.

Abell 520 resides 2.4 billion light-years away.