The longest stream of hydrogen gas ever seen has been discovered acting as a bridge between galaxies 500 light years away. 

The stream of atomic hydrogen measures in at 2.6 million light years long, beating the previous record holder in the Virgo cluster by almost a million light years. Despite spanning such a huge distance, the bridge actually utilised its immense size to evade detection until now.

“Its length actually makes it harder to detect – it has the same amount of gas as a massive galaxy, but spread over a much larger area,” explained Rhys Taylor, lead author of the study in the journal MNRAS.

“It’s a bit like moving a projector further away from a screen – the image gets bigger, but the ‘surface brightness’ (the emission per unit area) goes down, so it appears dimmer.”

The astronomers used the Arecibo radio telescope to survey the sky in search of hydrogen gas. “Hydrogen is thought to be the reservoir of fuel for star formation in galaxies, and we’re trying to understand the process which lead to star formation,” Taylor told the writer.

Streams of hydrogen are normally found engulfing galaxy clusters, but this newly discovered stream has only a few galactic neighbours. Its origin is still a puzzle, but the scientists have a number of theories to explain the unusual bridge.

One theory is that the large galaxy at one end of the bridge encountered the group of smaller galaxies in the past, dragging out the bridge as it moved away. Another idea is that the large galaxy smashed right through the smaller group, evicting the gas in the process.

Scientists are currently testing these theories using computer simulations which will eventually reveal the most likely scenario.