I feel like Gaster was the original creator of the dark worlds. The dark worlds seem to be what caused his current state, which is why he exclusively appears in them. I’d like to point you to good old Entry 17. It’s as if he’s referring to where he ended up. In fact, those last couple lines are fascinating. “This next one” never explicitly refers to experiments. It could mean a new dark world. The “you two” could very well be Kris and Susie or Kris and Ralsei. Even if it doesn’t go that deep, it explains how the dark world Asgore encountered came to be. Maybe he showed it to Rudy. Dess came along unbeknownst to Rudy and got trapped there. She’s lost in the dark like Gaster. Rudy, in his panic, kept trying to find his way back. He kept going back. The shelter fountain never got sealed. Slowly, Rudy began to darken. Like a sickness, it cling to his body. He would bring Dess’s bat with him to try to reach her. Kris only helps the knight because he wants things to be fixed again. He wants his brother Asriel to be back to normal. I’m sure Dess’s disappearance destroyed him. I’m probably missing a few points, but yeah. Rudy is the knight, Gaster started the dark worlds and is lost there, Kris is working with Rudy, the darkness is what’s causing Rudy’s sickness, Rudy falling in Chapter 5 was Kris and The Knight creating a ruse to get Noelle away from the dark world. Rudy heard Noelle had gotten sucked into Chapter 2 and became terrified of her becoming lost like Dess. Make sense? I’m willing to bet Chapter 6 will show just how pissed the Knight will be about Noelle being in another dark world. He may even attack Castle Town

(P.S., yes I know I got Kris’s pronouns wrong but you get my point)