Why does outer space look black?

Look up at the night sky with your own eyes, or marvel at images of the universe online, and you'll see the same thing: the inky, abysmal blackness of space, punctuated by bright stars, planets or spacecraft. But why is it black? Why isn't space colorful, like the blue daytime sky on Earth? 

Surprisingly, the answer has little to do with a lack of light. 

"You would think that since there are billions of stars in our galaxy, billions of galaxies in the universe and other objects, such as planets, that reflect light, that when we look up at the sky at night, it would be extremely bright," Tenley Hutchinson-Smith, a graduate student of astronomy and astrophysics at the University of California, Santa Cruz (UCSC), told Live Science in an email. "But instead, it's actually really dark." 

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Hutchinson-Smith said this contradiction, known in physics and astronomy circles as Olbers' paradox, can be explained by the theory of space-time expansion — the idea that "because our universe is expanding faster than the speed of light … the light from distant galaxies might be stretching and turning into infrared waves, microwaves and radio waves, which are not detectable by our human eyes." And because they are undetectable, they appear dark (black) to the naked eye.

Miranda Apfel, who is also a graduate student of astronomy and astrophysics at UCSC, agreed with Hutchinson-Smith. "Stars give off light in all colors, even colors not visible to the human eye, like ultraviolet or infrared," she told Live Science. "If we could see microwaves, all of space would glow." Apfel said this is because the cosmic microwave background — light energy from the Big Bang that was scattered by protons and electrons existing during the early universe — still fills all of space.

Another reason interstellar and interplanetary space appear dark is that space is a nearly perfect vacuum. Recall that Earth's sky is blue because molecules that make up the atmosphere, including nitrogen and oxygen, scatter a lot of visible light's component blue and violet wavelengths from the sun in all directions, including toward our eyes. However, in the absence of matter, light travels in a straight line from its source to the receiver. Because space is a near-perfect vacuum — meaning it has exceedingly few particles — there's virtually nothing in the space between stars and planets to scatter light to our eyes. And with no light reaching the eyes, they see black.

That said, a 2021 study in The Astrophysical Journal suggests that space may not be as black as scientists originally thought. Through NASA's New Horizons mission to Pluto and the Kuiper Belt, researchers have been able to see space without light interference from Earth or the sun. The team sifted through images taken by the spacecraft and subtracted all light from known stars, the Milky Way and possible galaxies, as well as any light that might have leaked in from camera quirks. The background light of the universe, they found, was still twice as bright as predicted. 

The reasons for the additional brightness, which remain unknown, will be the focus of future studies. Until then, one thing seems likely: Space could very well be more "charcoal" than pitch-black.

Originally published on Live Science.