  Mirror matter has been proposed as a dark matter candidate. It has several
very attractive features, including automatic stability and darkness, the
ability to mimic the broad features of cold dark matter while in the linear
density perturbation regime, and consistency with all direct dark matter search
experiments, both negative (e.g. CDMS II) and positive (DAMA). In this paper we
consider an important unsolved problem: Are there plausible reasons to explain
why most of the mirror matter in spiral galaxies exists in the form of gaseous
{\it spheroidal} galactic halos around ordinary matter {\it disks}? We compute
an order-of-magnitude estimate that the mirror photon luminosity of a typical
spiral galaxy today is around $10^{44}$ erg/s. Interestingly, this rate of
energy loss is similar to the power supplied by ordinary supernova explosions.
We discuss circumstances under which supernova power can be used to heat the
gaseous part of the mirror matter halo and hence prevent its collapse to a
disk. The {\it macro}scopic ordinary-mirror asymmetry plays a fundamental role
in our analysis.
