  One of possible explanations of a faint narrow emission line at 3.5 keV
reported in our Galaxy, Andromeda galaxy and a number of galaxy clusters is the
dark matter made of 7 keV sterile neutrinos. Another signature of such sterile
neutrino dark matter could be fewer ionizing sources in the early Universe
(compared to the standard "cold dark matter" (CDM) scenario), which should
affect the reionization of the Universe. By using a semi-analytical model of
reionization, we compare the model predictions for CDM and two different models
of 7 keV sterile neutrino dark matter (consistent with the 3.5 keV line
interpretation as decaying dark matter line) with available observations of
epoch of reionization (including the final measurements of electron scattering
optical depth made by Planck observatory). We found that both CDM and 7 keV
sterile neutrino dark matter well describe the data. The overall fit quality
for sterile neutrino dark matter is slightly (with $\Delta \chi^2 \simeq 2-3$)
better than for CDM, although it is not possible to make a robust distinction
between these models on the basis of the given observations.
