Undertale + Deltarune: Fountain Theory

Okay, I think I'm cooking with this one.

To start, I think it's important to state that I believe Sock Muppet's "Writing On The Wall" theory is so well-written and well thought out that I take it to be almost completely canon. There are a few minor things in the video that I don't think are true (I don't think Alphys necessarily had a crush on Asgore in the past, moreso just wanted to impress him) but there are two main ideas he covers that I'll be taking as important details for my Fountain Theory:

1. The lab entries are written by two different authors, Gaster and Alphys.
2. Gaster's entry 17 is a video recording, made during an experiment of his.

If you haven't seen the original video I'm talking about, I highly recommend watching it, as it's a really great theory and is very well made.

Anyway, with that in consideration, let's start with my theory.

Deltarune itself is a dark world within the world of Undertale.

I believe I have a decent amount of evidence supporting this theory, but as always with these sorts of things, it is just a theory. I might be wrong. That's the whole fun of theory-making. Also, I am writing this about a month after the release of Deltarune Chapters 3+4; things may change. So, with those things in mind as well, let's start with the Goner Maker.

After playing Chapter 4 of Deltarune, something stuck out in my mind. At the very end of the chapter, once you've nearly defeated the titan, Kris and Susie dive inside of it to extinguish the creature with the power of the soul. As they're falling, I noticed that the background looked a little familiar.

It's very similar to the background during the Goner Maker sequence all the way at the start of Chapter 1.

In fact, the fountains of every chapter seem to have this background. It just sticks out more to me during the titan fight because it takes up nearly the entire screen.

The biggest difference is that there seems to be two layers moving in different directions in the dark fountains and the titan, whereas the Goner Maker's background seems to have layers just emerging from a central point outwards.This is important, and I'll bring it up again later!

It's also fairly common and agreed upon knowledge that it is Gaster speaking to us during the Goner Maker sequence. He speaks in a very similar tone and style that we're used to seeing from Gaster. Wherever we are during this sequence, we are being guided by Gaster, before being abruptly ejected into the world of Deltarune.
All of this is to build my first main idea of Fountain Theory:

The Goner Maker sequence takes place within a dark fountain, guided by Gaster.

This idea isn't that revolutionary, and I think I've heard it mentioned before Chapters 3+4 came out. However, it's a major part of Fountain Theory and is a good starting point.

Next, let's take a dive back into the world of Undertale.
One of the very few things we know about Gaster within the world of Undertale is that he "fell into his creation", as stated by one of his goner followers.

The context of this moment seems to imply that Gaster fell into the CORE, however I'm not convinced that's the case.
Another one of his followers states that he was "shattered across time and space".

If he had fallen into the CORE, I'm not sure how he'd be shattered across time and space. The CORE is used as a power source for the underground. This explanation comes directly from Alphys, so I feel like there's not much mystery surrounding the core and its capabilities.My point is that I don't think Gaster fell into the CORE. I think he created and fell into something else.

Now seems like a good time to talk about lab entry 17. Here's a refresher if it's been a while.

ENTRY NUMBER
SEVENTEEN

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?

At the beginning of Deltarune Chapter 3, Ralsei tells Susie how Dark Worlds form. Specifically, when things get "darker than dark" and you "take away the light that wasn't there" you find yourself in the Dark World. The language here pretty closely parallels Gaster's entry 17. If you take away something from nothing, you get negative values... in a dark world, photon readings would be negative. Not zero. This leads into my next main idea behind Fountain Theory.

Gaster discovered and experimented with dark fountains within the world of Undertale, and eventually fell into one.

If we're assuming entry 17 is a video, then based on his vocabulary Gaster is speaking fairly coherently and confidently in it. I don't think he's "shattered" yet or is about to die. This contradicts part of Sock Muppet's theory, where he says that Gaster's on the brink of death here due to the shaking of the text in entry 17. I think this could be the case, but I think it's more likely that he's excited about his discovery, his creation, his dark fountain...

...his Deltarune.

Here's where my theory fully forms. From here on I'll be fleshing it out. To summarize what I believe happened:

Gaster, perhaps through his experiments with Determination and/or searching for a method of escape from the Underground, creates a dark fountain. He records his entry 17 before entering the dark world that his dark fountain has created, or perhaps as the dark world is taking form. Upon witnessing the nature of the world that has formed (which we know as the game Deltarune) Gaster falls in love with it, and affectionately names it "Deltarune" based on the symbol and prophecy from his own world of Undertale. From there he begins his experimentation on Deltarune, long before we eventually arrive to participate in it.
Then, at some later unknown time, he shatters across time and space, as stated by one of his followers. This is where the beginning of my theory comes back into play.

I believe Gaster shattered within the actual dark fountain that gives Deltarune form.

I'll be the first to admit that this part of my theory is a bit shaky. I'm not sure how Gaster shattered, or why, but somehow it led to him being shattered across time and space.

Remember when I said earlier that the layers of the fountains and the titan would be important? I think that the Goner Maker takes place within the dark fountain that gives Deltarune form, and the further from that dark fountain we go, the more these layers shift and move in different directions. I also think that the Goner Maker being in the main dark fountain is what allows us to both commune with Gaster so easily and create a vessel. Since it's where Gaster shattered, it's probably easier to interact with Gaster there, and since that fountain gives the whole world of Deltarune form, something like a vessel could be created there.