Astronomers are pointing to a hydrogen gas "bridge" spotted between two neighbour galaxies as strong evidence indicating both gravitationally bound systems had a close brush with one another billions of years ago.

The discovery of massive amounts of hydrogen gas streaming between two of our own Milky Way's neighbour galaxies — the Andromeda and the Triangulum — was disputed by some scientists back in 2004, when astronomers Robert Braun and David Thilker first noticed it via a telescope in the Netherlands.

The radio emission was so faint that most radio telescopes wouldn't be able to detect it, experts said.

But Jay Lockman, of the National Radio Astronomy Observatory, said in a release that further study with the National Science Foundation's Green Bank Telescope has since confirmed the existence of such a hydrogen bridge, supporting the theory that the two galaxies "may have passed close together in the distant past."

Six dense clumps of gas in the stream were identified by the advanced telescope.

Lockman added that studying the gaseous link "can give us a new key to understanding the evolution of both galaxies."

'Tidal tail'

The Andromeda is about 2.6 million light years from Earth; the Triangulum about three million light years away. Both are part of the so-called "Local Group" of galaxies that includes the Milky way and dozens of other galactic bodies.

Spencer Wolfe, of West Virginia University, said it's likely the hydrogen gas seen between the two galaxies was the remnant of a "tidal tail" of gas that streamed out after the close encounter.

"The encounter had to be long ago, because neither galaxy shows evidence of disruption today," he said in a release.

Lockman, Wolfe and two other scientists with West Virginia University and the University of Maryland recently presented their findings at the American Astronomical Society's meeting in Anchorage, Alaska.

"We plan to use the advanced capabilities of the GBT to continue this work and learn more about both the gas and, hopefully, the orbital histories of the two galaxies," Lockman said in a release.