  We present a study of cosmological implications of generic dark matter
decays. We consider two-body and many-body decaying scenarios. In the two-body
case the massive particle has a possibly relativistic kick velocity and thus
possesses a dynamical equation of state. This has implications to the expansion
history of the universe. We use recent observational data from the cosmic
microwave background, baryon acoustic oscillations and supernovae Type Ia to
obtain constraints on the lifetime of the dark matter particle. We find that
for an energy splitting where more than 40% of the dark matter particle energy
is transferred to massless, relativistic particles in the two-body case, or
more than 50% in the many-body case, lifetimes less than the age of the
universe are excluded at more than 95% confidence. When the energy splitting
falls to 10% the lifetime is constrained to be more than roughly half the age.
