  We present a systematic experimental study along with theoretical modeling of
the energy transfer in an ensemble of closely-packed CdTe colloidal
nanocrystals identified as the F\"orster resonant energy transfer (FRET). We
prove that at low temperature of 4.2 K, mainly the ground dark exciton states
in the initially excited small-size (donor) nanocrystals participate in the
dipole-dipole FRET leading to additional excitation of the large-size
(acceptor) nanocrystals. The FRET becomes possible due to the weak admixture of
the bright exciton states to the dark states. The admixture takes place even in
zero magnetic field and allows the radiative recombination of the dark
excitons. An external magnetic field considerably enhances this admixture, thus
increasing the energy transfer rate by a factor of 2-3 in a field of 15T, as
well as the radiative rates of the dark excitons in the donor and acceptor
nanocrystals. The theoretical modeling allows us to determine the spectral
dependence of the probability for the NC to serve as a donor for larger
nanocrystals, to evaluate the energy transfer rates as well as to predict their
dependencies on the magnetic field, to describe the spectral shift of the
photoluminescence maximum due to the energy transfer and to reproduce the
experimentally observed spectral dependencies of the photoluminescence
recombination dynamics in the magnetic field.
