How dark is dark? This is not a Dr. Seuss riddle but a serious question astronomers have been probing for years. Their big opportunity came with the New Horizons spacecraft that is now far beyond the planets at more than 5.4 billion miles from Earth. It's far enough to be free of light-contamination from the glow of background dust in our solar system. This means the spacecraft was able to measure the darkness of the seemingly pitch-black deep space.

Astronomers found a residual glow coming from myriad background galaxies filling the universe. The good news is that there is no other source of the background light to befuddle astronomers. New Horizons accomplished a full inventory of visible light. Launched in January 2006, New Horizons flew by the dwarf planet Pluto in July 2015, before passing by a Kuiper Belt object, Arrokoth, in January 2019.