If you look up into the sky on a clear night, you would see thousands of stars.  There are surely many more that you would need a telescope to see.  But there are not stars everywhere.  You can zoom in further and further with bigger and bigger telescopes, until eventually you find gaps where you simply don’t see stars.  For a long time it was thought that the gaps were empty, until the Hubble telescope peered through the darkness by taking a 200 hour exposure of a supposedly empty patch of sky.  What it revealed was a universe full of...