  Operating with the recently proposed model of hidden sector of the Universe,
based on the idea of photonic portal, we consider a new possible astrophysical
process of weak photoproduction of sterile scalars from the cold dark matter
consisting of sterile Dirac fermions. Subsequently, these scalars weakly decay
into secondary couples of photons. Then, primary photons, if energetic enough
and interacting in sufficiently dense dark-matter configurations, may form
secondary-photon cascades appearing as a new astrophysical phenomenon. Primary
photons, probing the cold dark matter, may come e.g. from gamma bursts. We show
how the cross-section for the photoproduction and the rate for the decay can be
calculated (in the centre-of-mass frame and at rest, respectively, for
simplicity).
