When sustained for megayears (refs. ^1,2), high-power jets from supermassive black holes (SMBHs) become the largest galaxy-made structures in the Universe³. By pumping electrons, atomic nuclei and magnetic fields into the intergalactic medium (IGM), these energetic flows affect the distribution of matter and magnetism in the cosmic web^4-6 and could have a sweeping cosmological influence if they reached far at early epochs. For the past 50 years, the known size range of black hole jet pairs ended at 4.6-5.0 Mpc (refs. ^7-9), or 20-30% of a cosmic void radius in the Local Universe¹⁰. An observational lack of longer jets, as well as theoretical results¹¹, thus suggested a growth limit at about 5 Mpc (ref. ¹²). Here we report observations of a radio structure spanning about 7 Mpc, or roughly 66% of a coeval cosmic void radius, apparently generated by a black hole between and 6.3 Gyr after the Big Bang. The structure consists of a northern lobe, a northern jet, a core, a southern jet with an inner hotspot and a southern outer hotspot with a backflow. This system demonstrates that jets can avoid destruction by magnetohydrodynamical instabilities over cosmological distances, even at epochs when the Universe was 7 to times denser than it is today. How jets can retain such long-lived coherence is unknown at present.