This very wide multi-frame panorama was taken in October 2014 at Canyon de Chelly National Monument in northeast Arizona. The zodiac light is on the left, the northern Milky Way on the right.

Z. Levay

While we can think of space as a vast sea of ​​darkness, all we have to do is look up at night to see it is punctuated by myriad stars, galaxies, and even a few Planets visible to the naked eye.

Scientists recently used data from NASA's New Horizons mission outside Pluto to measure how dark the cosmic background really is. What they found affects what we thought we knew about the composition of the entire universe.

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In short, space is so dark that there aren't as many galaxies to dim the background as astronomers previously estimated.

"It's an important number to know – how many galaxies are there?" Marc Postman of the Space Telescope Science Institute said in a statement Tuesday. "We just don't see light from 2 trillion galaxies."

This was the earlier estimate derived from observations made by the Hubble Space Telescope. However, a new study published in the Astrophysical Journal and co-authored by Postman suggests that the total number of galaxies in the universe is likely to be hundreds of billions rather than trillions.

Interestingly, this is closer to an even earlier number that suggests there were around 200 billion galaxies. This was based on Hubble data from the 1990s.

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Planetary nebula NGC 40


Because New Horizons is on the edge of the solar system, the surrounding sky is ten times darker than Hubble's.

"These types of measurements are extremely difficult. Many people have tried this for a long time," said study co-author Tod Lauer of the National Optical Infrared Astronomy Research Laboratory. "New Horizons gave us a point of view to measure the cosmic optical background better than anyone could before."

The team's results will be presented at a meeting of the American Astronomical Society on Wednesday.

The upcoming James Webb Space Telescopecurrently slated to launch on Halloween, could help provide further insight into how many and what types of galaxies provide the faint background light that keeps the universe from going completely pitch black.

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