Notes on Hidden Mirror World
  A few remarks on Dark Matter (DM) models are presented. An example is Mirror
Matter which is the oldest but still viable DM candidate, perhaps not in the
purest form. It can serve as a test-bench for other analogous DM models, since
the properties of macroscopic objects are quite firmly fixed for Mirror Matter.
A pedagogical derivation of virial theorem is given and it is pointed out that
concepts of virial velocity or virial temperature are misleading for some
cases. It is shown that the limits on self-interaction cross-sections derived
from observations of colliding clusters of galaxies are not real limits for
individual particles if they form macroscopic bodies. The effect of the heating
of interstellar medium by Mirror Matter compact stars is very weak but may be
observable. The effect of neutron star heating by accretion of M-baryons may be
negligible. Problems of MACHOs as Mirror Matter stars are touched upon.
