Diffuse Ionization in the Milky Way and Sterile Neutrinos
  We propose the radiative decay of sterile neutrinos which fill a fraction of the halo dark matter with a mass of 27.4 eV and lifetime of $\sim 10^{22}$ sec. as a way to explain the observed diffuse ionization in the Milky Way galaxy. Since the sterile neutrino number density in the present universe can be adjusted by arranging its dynamics appropriately, the resulting hot dark matter contribution to $\Omega_m$ can be small as required by many scenarios of structure formation. On the other hand a 27.4 eV neutrino could easily be the partial halo dark matter. One realization of this idea could be in the context of a mirror universe theory where the gauge and matter content of the standard model are completely duplicated in the mirror sector (except for an asymmetry in the weak scale); the three mirror neutrinos can mix with the known neutrinos via some strongly suppressed mechanism such as the gravitational or heavy right handed neutrino mediated forces. Two of the mirror neutrinos (say $\nu'_{\mu}$ and $\nu'_{\tau}$) could play the role of the above sterile neutrino. 
R. N. Mohapatra and D. W. Sciama