  Mirror matter is a stable self-collisional dark matter candidate. If parity
is a conserved unbroken symmetry of nature, there could exist a parallel hidden
(mirror) sector of the Universe composed of particles with the same masses and
obeying the same physical laws as our (visible) sector, except for the opposite
handedness of weak interactions. The two sectors interact predominantly via
gravity, therefore mirror matter is naturally "dark". Here I review the
cosmological signatures of mirror dark matter, concerning thermodynamics of the
early Universe, Big Bang nucleosynthesis, primordial structure formation and
evolution, cosmic microwave background and large scale structure power spectra.
Besides gravity, the effects on primordial nucleosynthesis of the kinetic
mixing between photons and mirror photons are considered. Summarizing the
present status of research and comparing theoretical results with
observations/experiments, it emerges that mirror matter is not just a viable,
but a promising dark matter candidate.
