The space between twinkling stars has commonly been thought to be filled with an jet-black void. But the question whether space itself expels a light of its own has stumped astronomers for decades – until now. US space agencyNASA’s New Horizons made headlines around the world when the probe in 2015 became the first to conduct a flyby of dwarf planet Pluto.

The craft’s unique position approaching the outer edge of our Solar System offers astronomers the unique opportunity to conduct rare experiments – in particular measuring of the Cosmic Optical Background (COB).

Hey, guys, we’re missing half of the stuff out there

Dr Michael Zemcov

This is the light from all sources outside the Milky Way galaxy spread diffusely throughout the observable Universe.

COB is the visible light analogue of the Cosmic Microwave Background (CMB), considered an important benchmark in astronomy.

By measuring this light, experts are able to discern stars’ positions, the size and density of galaxies, and even test theories about the structure and formation of the Universe itself.

As a result, understanding exactly how black the night sky is can provide profound insight into how the Universe forms and evolves.

NASA recently sought to determine once and for all whether there is a diffuse component to the COB (dCOB) – a source of light not associated with any currently known objects.

To detect the Universe’s own faint glow, researchers analysed images taken by the NASA’s New Horizons’ simple telescope and camera and simply searched for the most boring ones.

Dr Tod Lauer, an astronomer with the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab) in Arizona helped lead the research.

He said: ”The images were all of what you just simply call blank sky.

“There’s a sprinkling of faint stars, there’s a sprinkling of faint galaxies, but it looks random.

“What you want is a place that doesn’t have many bright stars in the images or bright stars even outside the field that can scatter light back into the camera.”

The least interesting of these images were then processed to remove all known sources of visible light.

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Once they had subtracted light from stars and camera quirks, the researchers were left with light arriving from beyond the galaxy.

The next step was to remove all light attributable to all known the galaxies, yet once that was accomplished, there was still plenty of unexplained light.

Dr Marc Postman, an astronomer with the Space Telescope Science Institute, said: “[Maybe there is] some other source of light that we don’t yet know what it is.”

Dr Michael Zemcov, a Rochester Institute of Technology astrophysicist, not involved with the research, added: “They’re saying that there’s as much light outside of galaxies as there is inside of galaxies, which is a pretty tough pill to swallow, frankly.

Dr Zemcov also analysed New Horizons data in a similar way four years ago and arrived at a similar conclusion.

He thinks astronomers have for centuries been studying visible light and yet somehow apparently “missed half the light in the Universe.”

He said: ”It’s very difficult to turn around and say to the astronomical community, like, ‘Hey, guys, we’re missing half of the stuff out there.’”

And the astronomer added: “I think the work is really solid.”

Candidates for explaining this explained light include an unknown phenomenon out in the Universe creating a visible spectrum.

And another possibility is something associated with dark matter – the elusive form of matter exerting a very real gravitational pull on visible matter.

Dr Postman said: ”As a person who studies the universe, I really want to know what the universe is made of and what are all the components of the Universe.

“We would like to think that the components that give off light are something that we can really get a good sense of and understand why there is that much light.”

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