Decades after the first evidence of non-luminous matter component, the nature of dark matter is still controversial at best. In this work we shine \textit{light} on the mysterious nature of \textit{dark} matter by probing the stability of the dark matter particle. We constraint the decay rate of the Cold Dark Matter (CDM) using latest Planck CMB and lensing measurement together with Baryonic Acoustic Oscillations (BAO) measurement from SDSS DR7, BOSS DR12, eBOSS DR16, and 6dFGS. In our baseline model all CDM is unstable and decays into radiation-like, dark products. We
then explore how changing the fraction of unstable to stable CDM affects the constraints on the decay rate. We find the most stringent constraints on dark matter decay rate for our baseline model of $\Gamma_\text{DCDM} < 0.129 \times 10^{-18}\,\mathrm{s}^{-1}$ (or, equivalently, the dark matter lifetime $\tau\text{DCDM} > 246$ Gyr) at 95\% C.L. with the combination of CMB, lensing and BAO measurements. We further conclude that the cosmological tension in the estimated values of H$_0$ and $\sigma_8$ persist in a universe with decaying dark matter.