  The imprint of decaying dark matter (DM) particles in characteristics of the
"21 cm fores" -- absorptions in 21 cm from minihalos in spectra of distant
radio-loud sources -- is considered within a 1D self-consistent hydrodynamic
description of minihalos from their turnaround point to virialization. The most
pronounced influence of decaying DM on evolution of minihalos is found in the
mass range $M=10^5-10^6\msun$, for which unstable DM with the current upper
limit of the ionization rate $\xi_{L} = 0.59\times 10^{-25}$ s$^{-1}$ depresses
21 cm optical depth by an order of magnitude compared to the standard
recombination scenario. Even rather a modest ionization $\xi \sim 0.3\xi_L$
practically "erases" absorption features and results in a considerable decrease
(by factor of more than 2.5) of the number of strong ($W_\nu^{obs} \simgt 0.3$
kHz at $z\simeq 10$) absorptions. At such circumstances broad-band observations
are to be more suitable for inferring physical conditions of the absorbing gas.
X-ray photons from stellar activity of initial episodes of star formation can
compete the contribution from decaying DM only at $z<10$. Therefore, when
observed the 21 cm signal will allow to follow evolution of decaying DM
particles in the redshift range $z=10-15$. Contrary, in case of non-detection
of the 21 cm signal in the frequency range $\nu<140$ MHz a lower limit on the
ionization rate from decaying dark matter can be established.
