As much as 99% of the gravitational energy liberated in a core collapse supernova emerges as kinetic energy of neutrinos. The neutrinos are produces in nuclear reactions (electron capture on nuclei and free protons) and non-nuclear reactions (e.g. pair annihilation $\gamma\to\text{e}^-+\text{e}^+\to\nu+\overline\nu$).
How can we estimate without detailed model calculations that as much as 99% of the gravitational energy (not, say, 50 or 10%) emerges as kinetic energy of neutrinos?
The 99% value is calculated from comparing maths to observations. In particular, you compute the gravitational potential energy of the precursor, $$ U\sim\frac{GM^2}{R}\sim10^{53}\,\text{erg} $$ Then you compare this to the observed energies in supernovae, which is typically $10^{51}$ erg (sometimes called a foe for fifty one ergs). Hence, the need for something that accounts for the unaccounted 99% of the energy.
That neutrinos could be the source of the missing energy wasn't fully confirmed until SN1987A, though it was theorized in the 60s and 70s.
