Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, were never detected. I will discuss the possibility of measurements of these seed magnetic fields with gamma-ray telescopes, via (non)observation of signal from electromagnetic cascades initiated by very-high-energy gamma-rays propagating through the intergalactic medium. First observational constraints on the intergalactic magnetic fields derived from the gamma-ray observations indicate that the seed fields in the intergalactic medium have cosmological origin.