The nucleus is an extraordinarily complex object where fundamental forces are at work. The solution of this many-body problem has challenged physicists for decades: several models with complementary virtues and flaws have been adopted, none of which has a universal predictive capability. Double beta decay is a second order weak nuclear decay whose precise measurement might steer fundamental improvements in nuclear theory. Its knowledge paves the way to a much better understanding of many body nuclear dynamics and clarifies, in particular, the role of multiparticle states. This is a useful input to a complete understanding of the dynamics of neutrino-less double beta decay, the chief physical process whose discovery may shed light to matter-antimatter asymmetry of the universe and unveil the true nature of neutrinos. Here, we report the study of 2νββ-decay in 82 Se with the CUPID-0 detector, an array of ZnSe crystals maintained at a temperature close to'absolute zero'in an ultralow background environment. Thanks to the unprecedented accuracy in the measurement of the two electrons spectrum, we prove that the decay is dominated by a single intermediate state. We obtain also the most precise value for the 82 Se 2νββ-decay half-life of $ T_ {1/2}^{2\nu}\,=\,\,[8.6\,\begin {array}{*{20}{c}}{+ 0.2}\\{-0.1}\end {array}]\,\,\times\,{10^{19}} $