  The influence of dark matter particle decay on the baryon-to-photon ratio has
been studied for different cosmological epochs. We consider different parameter
values of dark matter particles such as mass, lifetime, the relative fraction
of dark matter particles. It is shown that the modern value of the dark matter
density $\Omega_{\rm CDM}=0.26$ is enough to lead to variation of the
baryon-to-photon ratio up to $\Delta \eta / \eta \sim 0.01 \div 1$ for decays
of the particles with masses 10 GeV $\div$ 1 TeV. However, such processes can
also be accompanied by emergence of an excessive gamma ray flux. The
observational data on the diffuse gamma ray background are used to making
constraints on the dark matter decay models and on the maximum possible
variation of the baryon-to-photon ratio $\Delta\eta/\eta\lesssim10^{-5}$.
Detection of such variation of the baryon density in future cosmological
experiments can serve as a powerful means of studying properties of dark matter
particles.
