Wobbly white blobs.
That was all he could see when he heard the name Gaster. It gave him a very strange feeling, unlike he had ever felt. A strange mix of safety yet disappointment and a little sadness. A sinking feeling of being let down. Sans had always found it very odd that this name made him feel like this. He had only read the name W.D. Gaster in history books. A genius who created the core. A former Royal Scientist. There were books written about his work, his theories and experiments but there was nothing about his personal life. Surely there were other monsters who had interacted with him. It seems most of his team had disappeared too. But surely the king must've known him. Asgore wouldn't say a word though.
Sans sighed and shut his book. This always happened recently. He was working on his PhD. He had always been very good at science and, with some encouragement from his younger brother, decided to do something with it. The king seemed oddly eager to have him in the royal labs, though Sans had no idea why. As far as he knew he had never met the king. Sans and Papyrus' past had always been somewhat of a mystery, even to Sans. Now for a long time, Sans hadn't cared much about it. It looked like too much effort to look into and both he and Papyrus were currently content. Sans put his head into his hands. Well, that wasn't quite true anymore. Since starting his studies, since studying Gaster, Sans had started seeing things, dreaming things, maybe even remembering things. And it was all pushing him into his past. Especially after a very confusing feeling of Déjà vu he had the first time he went to the core. His first port of call had been Grillbz, the monster who had basically been guardian to Sans and Papyrus until Sans was old enough for the responsibility. He had been with them as long as Sans could remember, so surely he would know something about what was happening to him. However when Sans asked him, he immediately went silent. As soon as he mentioned the name Gaster, Grillbz looked angry. Sans had barely mentioned any of the things he had been seeing before Grillbz snapped at him to stop and made it very clear he wouldn't say a thing on the subject.
That had been nearly a year ago and these dreams had only gotten worse, spilling into daydreams and seeing things while he was awake. And all Sans had to go on was wobbly images, sentence fragments and the name Doctor W.D. Gaster.