W.D Gaster i have shared some things about him in another story people believe hes Sans Dad or old friend or something like that. SO apparently he had a experiment to see what happens if he kills off other timelines. Sans had told him it a was a bad idea Sans knew he would mess something up. Gaster didnt listen Sans pushed him into his own creation. Gaster ended up in a timeline that he had already killed. And it was called the "Void" And gaster has been stuck there forever since.