The observations were made with University of California-Los Angeles astronomer Michael Rich's relatively small 70-centimetre telescope in the US and using the 4.2-metre William Herschel telescope at La Palma, Spain.

After image processing, they saw an extremely faint stream more than 10 times the length of our Milky Way.

Named the Giant Coma Stream, it appeared to float in the middle of the cluster environment, not associated with any galaxy in particular, said the researchers in the paper published in the journal Astronomy & Astrophysics.

"This giant stream crossed our path by coincidence," said lead researcher Javier Roman, associated with the University of Groningen in the Netherlands and the University of La Laguna in Spain.

"We were studying halos of stars located around large galaxies."

The discovery of the Giant Coma Stream is remarkable because it is a rather fragile structure amid a hostile environment of mutually attracting and repelling galaxies.

"Meanwhile, we have been able to simulate such huge flows in the computer. We therefore expect to find more of them. For example, if we search with the future 39-metre ELT and when Euclid starts producing data," said co-author Reynier Peletier from University of Groningen.

With large future telescopes, the researchers not only hope to discover new giant streams, they also want to zoom in on the Giant Coma Stream itself.

"We would love to observe individual stars in and near the stream and learn more about dark matter," Peletier said.

The Coma Cluster is one of the best-studied clusters of galaxies. It contains thousands of galaxies at a distance of about 300 million light-years from Earth in the direction of the northern constellation Coma Berenices.

In 1933, Swiss astronomer Fritz Zwicky showed that the galaxies in the cluster move too fast if you only take the amount of visible matter into account. He figured out that there must be dark matter that keeps things together. The exact nature of dark matter is still unknown. IANS/KB