There are many ways to measure the extragalactic background (EBL), the universe’s glow from myriad sources. One of these measurement techniques uses the light from faraway blazars — active galaxies with supermassive black holes at their centers, blasting jets of high-speed particles and radiation out into space. Scientists study the gamma rays streaming from these high-powered jets. The gamma rays have a very long journey between their blazars and Earth, where astronomers can measure them. Along the way, some of these gamma rays will strike an intervening EBL photon and annihilate into a positron and an electron.
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