Talking Points Memo: Using NASA’s Fermi Gamma-Ray Space Telescope (Fermi), astronomers have measured the total extragalactic background light (EBL), the “cosmic fog” created by all the visible and UV photons emitted by all the stars that have ever existed. Launched in 2008, Fermi uses its primary instrument, the Large Area Telescope, to scan the entire sky for high-energy gamma rays emitted by blazars—distant galaxies with supermassive black holes. Because some of the gamma rays will collide with photons in the EBL and become electrons and positrons, the number of gamma rays emitted by a given source decreases over distance; hence, the more distant the blazar, the fewer high-energy gamma rays are detected by Fermi. By calculating the average gamma-ray attenuation across several distance ranges, astronomers were able to estimate the EBL’s thickness. They found the average stellar density to be about 1.4 stars per 100 billion cubic light-years, which means the average distance between stars is about 4150 light-years. Their paper was published online yesterday in Science Express.