  Helical hypermagnetic fields in the primordial Universe can produce the
observed amount of baryon asymmetry through the chiral anomaly without any
ingredients beyond the standard model of particle physics. While they generate
no $B-L$ asymmetry, the generated baryon asymmetry survives the spharelon
washout effect, because the generating process remains active until the
electroweak phase transition. Solving the Boltzmann equation numerically and
finding an attractor solution, we show that the baryon asymmetry of our
Universe can be explained, if the present large-scale magnetic fields indicated
by the blazar observations have a negative helicity and existed in the early
Universe before the electroweak phase transition. We also derive the upper
bound on the strength of the helical magnetic field, which is tighter than the
cosmic microwave background constraint, to avoid the overproduction of baryon
asymmetry.
