  I examine a possible spectral distortion of the Cosmic Microwave Background
(CMB) due to its absorption by galactic and intergalactic dust. I show that
even subtle intergalactic opacity of $1 \times 10^{-7}\, \mathrm{mag}\, h\,
\mathrm{Gpc}^{-1}$ at the CMB wavelengths in the local Universe causes
non-negligible CMB absorption and decline of the CMB intensity because the
opacity steeply increases with redshift. The CMB should be distorted even
during the epoch of the Universe defined by redshifts $z < 10$. For this epoch,
the maximum spectral distortion of the CMB is at least $20 \times 10^{-22}
\,\mathrm{Wm}^{-2}\, \mathrm{Hz}^{-1}\, \mathrm{sr}^{-1}$ at 300 GHz being well
above the sensitivity of the COBE/FIRAS, WMAP or Planck flux measurements. If
dust mass is considered to be redshift dependent with noticeable dust abundance
at redshifts 2-4, the predicted CMB distortion is even higher. The CMB would be
distorted also in a perfectly transparent universe due to dust in galaxies but
this effect is lower by one order than that due to intergalactic opacity. The
fact that the distortion of the CMB by dust is not observed is intriguing and
questions either opacity and extinction law measurements or validity of the
current model of the Universe.
