How dark is the sky, and what does that tell us about the number of galaxies in the visible universe? Astronomers can estimate the total number of galaxies by counting everything visible in a Hubble deep field and then multiplying them by the total area of the sky. But other galaxies are too faint and distant to directly detect. Yet while we can’t count them, their light suffuses space with a feeble glow.

To measure that glow, astronomical satellites have to escape the inner solar system and its light pollution, caused by sunlight reflecting off dust. A team of scientists has used observations by NASA’s New Horizons mission to Pluto and the Kuiper Belt to determine the brightness of this cosmic optical background. Their result sets an upper limit to the abundance of faint, unresolved galaxies, showing that they only number in the hundreds of billions, not 2 trillion galaxies as previously believed.

How dark does space get? If you get away from city lights and look up, the sky between the stars appears very dark indeed. Above the Earth’s atmosphere outer space dims even further, fading to an inky pitch-black. And yet even there, space isn’t absolutely black. The universe has a suffused feeble glimmer from innumerable distant stars and galaxies.