  The scenario that some first-order phase transitions may have taken place in
the early Universe offers us one of the most intriguing and fascinating
questions in cosmology. Indeed, the role played by the latent "heat" or energy
released in the phase transition is highly nontrivial and may lead to some
surprising, important results. In this paper, we take the wisdom that the
cosmological QCD phase transition, which happened at a time between 10^(-5) sec
and 10^(-4) sec or at the temperature of about 150 MeV and accounts for
confinement of quarks and gluons to within hadrons, would be of first order. To
get the essence out of the scenario, it is sufficient to approximate the true
QCD vacuum as one of degenerate theta-vacua and when necessary we try to model
it effectively via a complex scalar field with spontaneous symmetry breaking.
We examine how and when "pasted" or "patched" domain walls are formed, how long
such walls evolve in the long run, and we believe that the significant portion
of dark matter could be accounted for in terms of such domain-wall structure
and its remnants. Of course, the cosmological QCD phase transition happened in
the way such that the false vacua associated with baryons and many other
color-singlet objects did not disappear (that is, using the bag-model language,
there are bags of radius 1.0 fermi for the baryons) - but the amount of the
energy remained in the false vacua is negligible. The latent energy released
due to the conversion of the false vacua to the true vacua, in the form of
"pasted" or "patched" domain walls in the short run and their numerous evolved
objects, should make the concept of the "radiation-dominated" epoch, or of the
"matter-dominated" epoch to be re-examined.
