When you look at a galaxy cluster, most of the starlight comes from the galaxies themselves. But if you look deep enough, you will find a much fainter glow: the intracluster light (ICL). These are stars that no longer belong to any one galaxy, drifting freely in the space between them. Once seen as little more than an astronomical curiosity, ICL is now emerging as a powerful probe of how galaxy clusters grow and how dark matter is distributed.
