From the article:

"Stellar streams are elongated threads of gravitationally entwined stars that have likely been ripped away from their parent galaxies or nebulas by the gravitational pull of other nearby galaxies. Scientists have mapped dozens of these streams within galaxies, including the Milky Way. But until now, none had been discovered in intergalactic space, meaning the space between galaxies.

In the study, which was published Nov. 30 in the journal Astronomy & Astrophysics, the researchers identified and mapped the first-ever intergalactic stellar stream, which stretches through the Coma Cluster, also known as Abell 1656, a group of more than 1,000 small galaxies located around 321 million light-years from Earth. The researchers named the first-of-its-kind structure the Giant Coma Stream — so named because it is also the largest stellar stream ever found.

“This giant stream crossed our path by coincidence,” study lead author Javier Román, an astrophysicist at the Institute of Astrophysics of the Canary Islands, said in a statement. The team was initially studying halos of dispersed stars around the Coma Cluster, in an attempt to measure the dark matter that surrounds the galaxy group, when they came across the starry trail."