  If CDM particles decay and their lifetime is comparable to the age of the
Universe, they can modify its equation of state. By comparing the results of
numerical simulations with high redshift SN-Ia observations we show that this
hypothesis is consistent with present data. Fitting the simplest quintessence
models with constant $w_q$ to data leads to $w_q \lesssim -1$. We show that a
universe with a cosmological constant or quintessence matter with $w_q \sim -1$
and a decaying Dark Matter has an effective $w_q < -1$ and fits SN data better
than stable CDM or quintessence models with $w_q > -1$.
