Several CP violating processes occurring in the early universe could have produced weak-field helical magnetic seed fields that would have subsequently grown, amplified via different types of dynamo mechanism, accounting for the presently observed astronomical-sized fields. I will describe causal mechanisms for the generation of seed fields at the time of the electroweak phase transition, and show how the structure of these large-scale magnetic fields is imprinted on the gamma-ray sky through secondary cascades. Data from the Fermi satellite supports the presence of a left-handed helical magnetic field of $10^{-14}G$ on $~10$ Mpc scales.