In the mid-20th century, Penzias and Wilson discovered a faint all-pervading glow left over after the birth of the Universe, and Hubble proved its expansion, which stretches the light. But sometimes, rays emitted by vortices around black holes, star explosions, restless galaxy cores, or stellar nurseries travel through near-emptiness and collide with the ancient glow. Like two flashlight beams crossing in the fog, they produce a spark, but instead of a spark, a pair emerges: an electron and its anti-twin, the positron.

According to E=mc², light energy transforms into matter. The newborn particles immediately emit gamma radiation, and myriads of such flashes pour into the overall background. Calculations show that the peak of these transformations occurred when the Universe was four times smaller (astronomers would say redshift 2.7). The most amazing part: although in a single galaxy the event happens once every billion years, on cosmic scales these rare sparks add up to a noticeable glow, explaining up to 20% of the gamma-ray sky — without invoking mysterious dark matter.