https://www.vice.com/en_us/article/akvabe/a-vast-faint-universe-of-rogue-stars-is-revealing-dark-matter

Astronomers are learning to decipher clues imprinted in the ultra-faint light of free-floating stars that have been kicked out of their galaxies and left to wander the vast interstitial realms of space. 

This ambient glow, known as intracluster light because it shines between galaxies in clusters, is a powerful new way to probe longstanding mysteries, such as the nature of dark matter, an unidentified substance that is abundant in the universe.