Mirror baryons as the dark matter

A "parallel world" with all of the standard particles, which primarily interacts gravitationally with our world, can be motivated via a symmetry principle designed to make a Lagrangian $\mathrm{CP}$ symmetric, while maintaining a $\mathrm{CP}$ asymmetry in the observable world. Such a symmetry is easily accommodated in grand unified theory models, and may also arise in superstring theories. The cosmological abundance of mirror particles is investigated after a period of chaotic inflation and subsequent reheating. Contrary to previous studies, I find that mirror and ordinary abundances may naturally be similar at the present epoch, and that mirror baryons can provide the closure density without violating nucleosynthesis constraints.