>>528369470
he was studying determination and injecting it into various things. Along the way he discovered how to make purer and purer darkness as a byproduct. Presumably from injecting determination into inanimate objects and/or the ground itself
they say he 'fell into his invention' but that could just be what sans and papyrus believe happened, or even just the way sans decided to tell it. Could be at the bottom of his machine he opened a dark fountain and fell into the depths. could be he turned himself into a hyper darkner of sorts, a being too far removed from the light to actually be seeable except in conditions with negative amounts of light. Either way, his existence got scattered across the macrocosm