  Possible detection of signatures of structure formation at the end of the
'dark age' epoch (z~40-20) is examined. We discuss the spectral-spatial
fluctuations in the CMBR temperature produced by elastic resonant scattering of
CMBR photons on HD molecules located in protostructures moving with peculiar
velocity. Detailed chemical kinematic evolution of HD molecules in the
expanding homogeneous medium is calculated. Then, the HD abundances are linked
to protostructures at their maximum expansion, whose properties are estimated
by using the top-hat spherical approach and the LambdaCDM cosmology. We find
that the optical depths in the HD three lowest pure rotational lines for
high-peak protohaloes at their maximum expansion are much higher than those in
LiH molecule. The corresponding spectral-spatial fluctuation amplitudes however
are probably too weak as to be detected by current and forthcoming
millimeter-telescope facilities. We extend our estimates of spectral-spatial
fluctuations to gas clouds inside collapsed CDM haloes by using results from a
crude model of HD production in these clouds. The fluctuations for the
highest-peak CDM haloes at redshifts ~20-30 could be detected in the future.
Observations will be important to test model predictions of early structure
formation in the universe.
