We present upper limits on line emission in the Cosmic X-ray background ( CXB ) that would be produced by decay of sterile neutrino dark matter .
We employ the spectra of the unresolved component of the CXB in the Chandra Deep Fields North and South obtained with the Chandra CCD detector in the E = 0.8 - 9 keV band .
The expected decay flux comes from the dark matter on the lines of sight through the Milky Way galactic halo .
Our constraints on the sterile neutrino decay rate are sensitive to the modeling of the Milky Way halo .
The highest halo mass estimates provide a limit on the sterile neutrino mass of m _ { s } < 2.9 keV in the Dodelson-Widrow production model , while the lowest halo mass estimates provide the conservative limit of m _ { s } < 5.7 keV ( 2 \sigma ) .
We also discuss constraints from a short observation of the softer ( E < 1 keV ) X-ray background with a rocket-borne calorimeter by McCammon and collaborators .