  We analyze the formation of the signal in the 21 cm hydrogen line of the Dark
Ages ($30\le z\le300$) in different cosmological models and discuss the
possibility of its detection by decameter-wavelength radio telescopes. To study
the dependence of the intensity and profile of the line on the values of
cosmological parameters and physical conditions in the intergalactic medium,
the evolution of the global (averaged over the sky) differential brightness
temperature in this line was calculated in standard and non-standard
cosmological models with different parameters. The standard $\Lambda$CDM model
with cosmological parameters predicts a value of the differential brightness
temperature in the center of the absorption line $\delta T_{br}\approx-35$ mK
at $z\approx87$. The frequency of the line at the absorption maximum is 16 MHz,
the effective width of the line is $\approx$25 MHz. The depth of the line is
moderately sensitive to $\Omega_b$ and $H_0$, weakly sensitive to $\Omega_{dm}$
and insensitive to other parameters of the standard $\Lambda$CDM model.
However, the line is very sensitive to additional mechanisms of heating or
cooling of baryonic matter during the Dark Ages, so it can be an effective test
of non-standard cosmological models. In models with decaying and
self-annihilating dark matter, as well as with an initial global stochastic
magnetic field, the temperature of baryonic matter in this period is higher,
the higher the density of these dark matter components and the magnetic field
strength. The absorption line becomes shallower, disappears, and turns into
emission at values of the component parameters lower than the upper bounds on
them, which follow from observational data. The estimations show that such
spectral features can be detected by decameter-wavelength radio telescopes in
the near future.
