  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 the matter-antimatter
asymmetry of the universe and unveil the true nature of neutrinos. Here, we
report the study of $2\nu\beta\beta$-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 $2\nu\beta\beta$-decay half-life
of $T^{2{\nu}}_{1/2} = [8.6^{+0.2}_{-0.1}] \times 10^{19}$ yr.
