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 the 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 .