When the New Horizons probe reached the outer dark of the Solar System, out past Pluto, its instruments picked up something strange.
Very, very faintly, the space between the stars was glowing with optical light. This in itself was not unexpected; this light is called the cosmic optical background, a faint luminescence from all the light sources in the Universe outside our galaxy.
The strange part was the amount of light. There was significantly more than scientists thought there should be – twice as much, in fact.
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