Ya know the shadow crystals? What if those are pieces of Gaster? Gaster was shattered across space and time, and we don't really know what he is. The shadow crystals are made of a mysterious substance and are dark, referencing Gaster's thing about "darker, yet darker." Gaster might be a "hallucination" caused by the shadow crystals that only their holders can see, as when the Addisons tried to see who was on the phone with Spamton, there was nothing but garbage noise. It would be funny if this theory was true, as the shadow crystals stay pretty similar in the light world, meaning that they are probably something strange and important.