  A possible explanation for the enigma of dark energy, responsible for about
76 % of the mass-energy of the universe, is obtained by requiring only that the
rigorous continuity equation (the Boltzmann transport equation) for quanta
propagating through space should have the form of a Lorentz-covariant and
dispersion-free wave equation. This requirement implies (i) properties of
space-time which an observer would describe as uniform expansion in agreement
with Hubble's law, and (ii) that the quantum transport behaves like in a
multiplicative medium with multiplication factor = 2. This inherent,
essentially explosive multiplicity of vacuum, thus caused by the requirement of
Lorentz-covariance, is suggested as a potential origin of dark energy. In
addition, it is shown (iii) that this requirement of Lorentz-covariant quantum
transport leads to an apparent accelerated expansion of the universe.
  In addition to the updated manuscript for Phys. Scr., the arXiv file also
contains a discussion of about a dozen Frequently Asked Questions from readers.
Further questions not adequately dealt with in the existing FAQ are welcome.
