  We present the extended electrodynamics in the presence of the axion and dark
photon. We derive the extended versions of Maxwell's equations and dark
Maxwell's equations (for both massive and massless dark photons) as well as the
wave equations. We discuss the implications of this extended electrodynamics
including the enhanced effects in the particle conversions under the external
magnetic or dark magnetic field. We also discuss the recently reported anomaly
in the redshifted 21cm spectrum using the extended electrodynamics.
