This plot shows the locations of 150 blazars (green dots) used in the EBL study. The background map shows the entire sky and was constructed from four years of gamma-rays with energies above 10 billion electron volts (GeV) detected by Fermi. The plane of our Milky Way Galaxy runs along the middle of the plot. The Fermi LAT instrument is the first to detect more than 500 sources in this energy range. // Credit: NASA/DOE/Fermi LAT Collaboration