  If dark matter decays to electromagnetically-interacting particles, it can
inject energy into the baryonic gas and thus affect the processes of
recombination and reionization. This leaves an imprint on the cosmic microwave
background (CMB): the large-scale polarization is enhanced, and the small-scale
temperature fluctuation is damped. We use the WMAP three-year data combined
with galaxy surveys to constrain radiatively decaying dark matter. Our new
limits to the dark-matter decay width are about ten times stronger than
previous limits. For dark-matter lifetimes that exceed the age of the Universe,
a limit of $\zeta \Gamma_{\chi} < 1.7 \times 10^{-25} s^{-1}$ (95% CL) is
derived, where $\zeta$ is the efficiency of converting decay energy into
ionization energy. Limits for lifetimes short compared with the age of the
Universe are also derived. We forecast improvements expected from the Planck
satellite.
