  A simple way to accommodate dark matter is to postulate the existence of a
hidden sector. That is, a set of new particles and forces interacting with the
known particles predominantly via gravity. In general this leads to a large set
of unknown parameters, however if the hidden sector is an exact copy of the
standard model sector, then an enhanced symmetry arises. This symmetry, which
can be interpreted as space-time parity, connects each ordinary particle ($e, \
\nu, \ p, \ n, \ \gamma, ....)$ with a mirror partner ($e', \ \nu', \ p', \ n',
\ \gamma', ...)$. If this symmetry is completely unbroken, then the mirror
particles are degenerate with their ordinary particle counterparts, and would
interact amongst themselves with exactly the same dynamics that govern ordinary
particle interactions. The only new interaction postulated is photon - mirror
photon kinetic mixing, whose strength $\epsilon$, is the sole new fundamental
(Lagrangian) parameter relevant for astrophysics and cosmology. It turns out
that such a theory, with suitably chosen initial conditions effective in the
very early Universe, can provide an adequate description of dark matter
phenomena provided that $\epsilon \sim 10^{-9}$. This review focuses on three
main developments of this mirror dark matter theory during the last decade:
Early universe cosmology, galaxy structure and the application to direct
detection experiments.
