Astronomers using ESA and NASA high-energy observatories have discovered a tantalising clue that hints at an elusive ingredient of our Universe: dark matter.

Although thought to be invisible, neither emitting nor absorbing light, dark matter can be detected through its gravitational influence on the movements and appearance of other objects in the Universe, such as stars or galaxies.

Based on this indirect evidence, astronomers believe that dark matter is the dominant type of matter in the Universe – yet it remains obscure.

Now a hint may have been found by studying galaxy clusters, the largest cosmic assemblies of matter bound together by gravity.

Galaxy clusters not only contain hundreds of galaxies, but also a huge amount of hot gas filling the space between them.

However, measuring the gravitational influence of such clusters shows that the galaxies and gas make up only about a fifth of the total mass – the rest is thought to be dark matter.

The gas is mainly hydrogen and, at over 10 million degrees celsius, is hot enough to emit X-rays. Traces of other elements contribute additional X-ray ‘lines’ at specific wavelengths.

Examining observations by ESA’s XMM-Newton and NASA’s Chandra spaceborne telescopes of these characteristic lines in 73 galaxy clusters, astronomers stumbled on an intriguing faint line at a wavelength where none had been seen before.