  We use a finite temperature effective field theory recently developed for
superfluid Fermi gases to investigate the properties of dark solitons in these
superfluids. Our approach provides an analytic solution for the dip in the
order parameter and the phase profile accross the soliton, which can be
compared with results obtained in the framework of the Bogoliubov - de Gennes
equations. We present results in the whole range of the BCS-BEC crossover, for
arbitrary temperatures, and taking into account Gaussian fluctuations about the
saddle point. The obtained analytic solutions yield an exact energy-momentum
relation for a dark soliton showing that the soliton in a Fermi gas behaves
like a classical particle even at nonzero temperatures. The spatial profile of
the pair field and for the parameters of state for the soliton are analytically
studied. In the strong-coupling regime and/or for sufficiently high
temperatures, the obtained analytic solutions match well the numeric results
obtained using the Bogoliubov - de Gennes equations.
