The influence of ultra-high energy cosmic rays ( UHECRs ) and decaying dark matter particles on the emission and absorption characteristics of neutral hydrogen in 21 cm at redshifts z = 10 - 50 is considered .
In presence of UHECRs 21 cm can be seen in absorption with the brightness temperature T _ { b } = - ( 5 - 10 ) mK in the range z = 10 - 30 .
Decayng particles can stimulate a 21 cm signal in emission with T _ { b } \sim 50 - 60 mK at z = 50 , and T _ { b } \simeq 10 mK at z \sim 20 .
Characteristics of the fluctuations of the brightness temperature , in particular , its power spectrum are also calculated .
The maps of the power spectrum of the brightness temperature on the plane wavenumber-redshift are shown to be sensitive to the parameters of UHECRs and decaying dark matter .
Observational possibilities to detect manifestations of UHECRs and/or decaying particles in 21 cm with the future radio telescopes ( LOFAR , 21CMA and SKA ) , and to distinguish contributions from them are briefly discussed .