 Intergalactic light, also known as intracluster light or ICL for short, is the light from stars that exist outside of galaxies, in the space between galaxies in a galaxy cluster. These stars are not gravitationally bound to any one particular galaxy in their cluster. They were first detected in the coma galaxy cluster in 1951 by Fritz Zwicky. He reported that one of his most interesting discoveries was observing luminous, faint intergalactic matter in the cluster. Here's a Hubble image. The artificially added blue color is where Hubble data found this intracluster light. The extremely faint glow traces a smooth distribution of light from stars scattered across the cluster. Astronomers are trying to find out how these stars came to be where they are. There are two competing theories. One is that the stars formed inside galaxies and were stripped away by galaxy cluster gas or by galaxy collisions. The competing theory is that these stars actually formed outside of any galaxy at all. Here's a second galaxy out of the tens studied for this project. If the first theory is correct, we would expect the percentage of light generated by intergalactic stars to grow over time as more and more of the stars are produced. But this Hubble survey shows that intergalactic light accounts for around 17% of a cluster's light, no matter how old the cluster is. This suggests that these stars formed at the same time as their galaxy clusters formed. These findings have significant implications for dark matter distribution analysis and the calculations for gravitational lensing so critical for studies of the early universe.