  We compute the primordial mirror helium He' mass fraction emerging from Big
Bang nucleosynthesis in the mirror sector of particles in the presence of
kinetic mixing between photons and mirror photons. We explore the kinetic
mixing parameter (epsilon) values relevant for cosmology and which are also
currently probed by the dark matter direct detection experiments. In
particular, we find that for epsilon \sim 10^{-9}, as suggested by the
DAMA/Libra and other experiments, a large He' mass fraction (Y_{He'} \approx
90%) is produced. Such a large value of the primordial He' mass fraction will
have important implications for the mirror dark matter interpretation of the
direct detection experiments, as well as for the study of mirror star formation
and evolution.
