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We analyze the formation of the redshifted hyperfine structure line 21-cm ofhydrogen atoms in Dark Ages at $30\le z\le300$ in the different cosmologies. Tostudy its dependence on the values of cosmological parameters and physicalconditions in the intergalactic medium, the evolution of the global(sky-averaged) differential brightness temperature in this line was computed instandard and non-standard cosmological models with different parameters. Thestandard $\Lambda$CDM model with post-Planck parameters predicts a value of thedifferential brightness temperature in the center of the absorption line$\delta T_{br}\approx35$ mK at $z\approx87$. The frequency of the line in theabsorption maximum is 16 MHz, the effective half-width of the line is 17 MHz.The depth of line is moderately sensitive to $\Omega_b$ and $H_0$, weaklysensitive to $\Omega_{dm}$, and insensitive to other parameters of the standard$\Lambda$CDM model. But line is very sensitive to the additional mechanisms ofheating or cooling of baryonic matter during the Dark Ages, so it can be a goodtest of non-standard cosmological models. In the models with decaying andself-annihilating dark matter, as well as with a primordial stochastic magneticfield, the temperature of baryonic matter in this period is higher if thelarger is the fraction of these energy components of dark matter and magneticfield strength. The absorption line becomes shallower, desappers andtransitions to emission at values of the component parameters lower than theupper limits on them following from the current observational data. Estimatesshow that such spectral features may be detected by radio telescopes in thedecameter wavelength range in the near future.

Signal in the Hyperfine Structure Line of the Ground State of Atomic Hydrogen from the Dark Ages as a Cosmological Test