Scientists have used a NASA probe way out in space, beyond Pluto, to measure visible light that's not connected to any known source such as stars or galaxies.

Transcript

NOEL KING, HOST:

If you are outside of a city and it's a clear night, you'll see stars in the sky. And you'll see the space between the stars is dark. I'm not exactly blowing your mind here, am I? No. The question is, how dark is the dark space? Astronomers recently tried to find out. Here's NPR's Nell Greenfieldboyce.

NELL GREENFIELDBOYCE, BYLINE: The darkness of outer space seems like a pretty basic thing to know. But Tod Lauer says it's long been a mystery. He's with the National Science Foundation's National Optical Infrared Astronomy Research Laboratory. And he says, what would it look like if you could somehow take away all the stars and galaxies that give out visible light?

TOD LAUER: Is there any light coming from the universe itself? Is space truly black? Or is there a little bit of a glow that you can see?

GREENFIELDBOYCE: He says it's hard to detect a faint glow when there's lots of other bright light around. That's the case on Earth and all through the inner solar system, which is full of dust that reflects sunlight. Out beyond all the planets, however, that dust isn't a problem. The sunlight is much weaker. And right now, that's where you'll find NASA's New Horizons probe. Five years ago, this spacecraft made history when it flew past Pluto. Since then, it's just kept going. It's more than 4 billion miles from home. To try to see how dark space really is, Lauer and other members of the New Horizons team analyzed a bunch of images taken by the spacecraft, unexciting images of just blank, dark sky, maybe sprinkled with faint stars.

LAUER: So it's sort of boring images of fields with a few stars in them. But for us, that's gold to be mined.

GREENFIELDBOYCE: They processed the images to essentially just take away the light from all known sources. Marc Postman is an astronomer with the Space Telescope Science Institute in Baltimore, Md.

MARC POSTMAN: The main things you need to subtract off are scattered light from other nearby bright stars, light from very faint stars and very faint galaxies that we know about.

GREENFIELDBOYCE: Scattered light from the Milky Way itself, light from all the galaxies beyond our own that are thought to be out there. And he says, here's the thing. When they did all that, in the end, they were left with about a quarter of the light they started with. That means a quarter of the total light they measured in outer space could not be accounted for. Where did it come from? Postman says either there are twice as many galaxies as people think...

POSTMAN: Or there's some as yet unknown new source of light that we need to think about.

GREENFIELDBOYCE: Maybe even something related to dark matter. That's a mysterious form of matter that exerts a gravitational pull on visible matter but has never been seen directly.

MICHAEL ZEMCOV: I think if it was dark matter, that would be amazing.

GREENFIELDBOYCE: Michael Zemcov is an astrophysicist at Rochester Institute of Technology who wasn't part of the research team. He thinks the results seem solid, if disconcerting.

ZEMCOV: If you look at what they're saying, 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.

GREENFIELDBOYCE: He says that's because astronomers have been studying visible light for centuries but seem to be missing something pretty important.

ZEMCOV: Are we sure that the big galaxies are where everything is? Maybe it's a bunch of little stuff. Maybe it's some other exotic process that we don't know about yet.

GREENFIELDBOYCE: So how dark is outer space? There is light out there, light that scientists can't explain. But still, it's pretty dark.

Nell Greenfieldboyce, NPR News.

(SOUNDBITE OF SOLAR HEAVY'S "THE FALL") Transcript provided by NPR, Copyright NPR.

Tags: New Horizons spacecraft  Milky Way  universe