ok so heres my idea and its got a lotta harry potter involved.

so my idea is that the gaster followers were once normal people. until they chose to join gaster. like the dark mark to join gasters army you needed to show signs of it.

i'd assume in this case gaster would take all colour away from them removing part of their SOUL for a expreiment. the transformation stopped them from dying, feeling joy and make them forever loyal to gaster.

now one thing that may ruin that idea is that gaster was trusted by asgore and never really seemed to be plotting anything that asgore knew of.

my response? THE TRUE LAB!

what asgore may have seen was the lab that we now know as alphys lab. perhaps the true lab was always locked like with alphys. and nobody knew what went on down there.

now we get to the true lab bit.

so gasters expriment was going very strange from what entry 17 said. you all remember right? "Dark darker yet darker. The darkness keeps growing. The shadows cutting deeper. Photon readings negative. This next experiment seems very very interesting. What do you two think?"

what this tells me is that there were 2 people in the lab with him and if i had to bet all his followers were in that lab as to not raise suspicion over why all there were grey monsters now. not that nobody was supicious already. gaster always got messages over what happened to they're children (like maybe monster kids mom) gaster like voldermort would trick them instantly and easily.

so this experiment that we know gaster was doing was going good and all. but why did gaster not do anything evil? like kill other monsters? well you see i imagine that gaster would just want a part of their SOULS to make sure they will be loyal to him. i think that if that failed experiment never happened then gaster would've taken over.

it could've been as simple as this: gaster tells asgore he found a way to break the barrier and needed every monster to come right away. and one by one he would take a peice of their SOUL and combine them to make maybe mutiple human SOULS. since the experiment may have changed the SOULS power. after that he wouldn't only start a war with humans again but would take over the entire human world with his mionions and his newly powered souls.

but anyway. so what must've happend is that he was so close to his goal and called all his minions over to see.

however instead of what gaster wanted to happen there must've been a explosion. and a big one. it killed many of his followers. but some on them didn't die. and were just stuck in worlds where they may exist. may not.

its just like that monster kid gaster follower phrase "you ever wonder a world where you don't exist?"

and that would be there fates from now on. sometimes they would exist. and othertimes they wouldn't

i hope nobody takes this theory seriously. as its really all just me being stupid so cya!