This animation tracks several gamma rays through space and time, from
their emission in the jet of a distant blazar to their arrival in
Fermi's Large Area Telescope (LAT). During their journey, the number of
randomly moving ultraviolet and optical photons (blue) increases as more
and more stars are born in the universe. Eventually, one of the gamma
rays encounters a photon of starlight and the gamma ray transforms into
an electron and a positron. The remaining gamma-ray photons arrive at
Fermi, interact with tungsten plates in the LAT, and produce the
electrons and positrons whose paths through the detector allows
astronomers to backtrack the gamma rays to their source.

Credit: NASA's Goddard Space Flight Center/Cruz deWilde