During the missing years, Biff is paralyzed by the Buddhist nuns and it last a few days, with them making fun of him. Much later Biff manages to apply the paralysis ointment to Jesus' lips, making him appear to be dead for a while.
Edited by xongsmith, : questions end with question marks
You have heard about multiverses, yes? Imagine gazillions of them arising every planck time with all kinds of different atomic/subatomic constants. Most will vanish instantly, some will last a while longer and some will last billions of years. The Evolutionary Cosmology, sort of an offshoot of the Anthropomorphic Principle, would argue that we just happen to be in one that has these constants that we observe that create this chemistry.