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Magnetic fields in galaxies are produced via the amplification of seedmagnetic fields of unknown nature. The seed fields, which might exist in theirinitial form in the intergalactic medium, were never detected. We report alower bound $B\ge 3\times 10^{-16}$~gauss on the strength of intergalacticmagnetic fields, which stems from the nonobservation of GeV gamma-ray emissionfrom electromagnetic cascade initiated by tera-electron volt gamma-ray inintergalactic medium. The bound improves as $\lambda_B^{-1/2}$ if magneticfield correlation length, $\lambda_B$, is much smaller than a megaparsec. Thislower bound constrains models for the origin of cosmic magnetic fields.

Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars