she's killing 'em

For the sake of this theory, I will use "Anomaly" to describe any entity who has the power to save/reload. I know UT is ambiguous on who exactly the anomaly is, just bear with me.

Anyway, Sans says this during his boss fight:

"our reports showed a massive anomaly in the timespace continuum. timelines jumping left and right, stopping and starting…until suddenly, everything ends. heh heh heh…that's your fault, isn't it?"

In UT, timelines can't exactly jump "left and right", because you only have one save file. But in DR, you have multiple save files, so timelines can jump left and right. Furthermore, Toby said that what you do in DR will not affect UT. Finally, Sans' "Don't Forget" drawing in UT seems to have came from DR. It's safe to say DR is a prequel to UT.

What if DR was some kind of experiment that trapped an anomaly, to study their saving/reloading abilities? And that's why Sans knows so much about them. DR might not even be the first experiment done on anomalies, since even in DR, Sans seems to know more than he should; eg the bread crumbs thing in the Weird Route. That's why Gaster doesn't seem to care about you doing the Weird Route, because it still provides useful data on anomalies.

As for the nature of this experiment, I'm not really sure. My initial idea was this:

1. Hometown is just straight up a computer simulation

Device Theory is all but confirmed at this point, so Hometown just being a simulation running on Gaster's computer would be the simplest explanation. That's why Hometown seems to be isolated from the rest of the world: that's all that exists in the DR world. But there's a problem with that: the Egg scenes. The one in Chapter 3, with "Mancountry", implies Kris has repressed memories of meeting other humans. There are ways to rationalize that, but they feel kind of clunky to me: Like, maybe a SOUL is needed to power the simulation, and one of the six captured souls were used to power the device (before being brought to Asgore), and that SOUL's memories are leaking into DR, as Kris' memories...? Or maybe that captured SOUL (presumably the Cyan one) is plugged into the simulation as Kris...? I dunno. Feels clunky, and maybe even bleak. It would imply Kris' narrative arc is to escape DR and get murdered by UT!Asgore.

Also, only those with the most determination in a world can save/reload. Would a computerized world count as a world? Beats me. So I have another idea:

2. Gaster and his followers secretly invaded the world of DR, and trapped Hometown in stasis to facilitate this experiment

Less bleak and can be more easily justified as a "world", but still kind of clunky. I then realized the simplest explanation:

3. Hometown is a computer simulation based off an existing parallel universe

Anyway, three different flavors of explaining why Hometown seems isolated.

I think the Knight is bringing about the Roaring in hopes that the Titans can be somehow utilized to end the experiment and free Hometown from its isolation. If #2 is true, the Knight is doing it to free Hometown from the stasis imposed by Gaster. If #1/#3 is true, then perhaps the Knight is doing this so them and the rest of Hometown can finally become "real." I imagine it's a reckless, "the ends justify the means" plan. It would definitely parallel UT!Asgore promising to liberate monsterkind.

I also think this plan is doomed to fail, which is why Gaster isn't intervening to stop it. As for the Eggs, I think they're things replaced by Deltarune's Error Handler. I think the Eggs are objects that remind Kris of their life before they came to Hometown, which causes the experiment to glitch out. That's why they're so tied to Kris' trauma.

The Eggs are described as an issue, implying they're a real threat to the experiment. But they're also described as "not too important, not too unimportant." Given DR is supposed to only have one ending, this makes me think that every ending will lead to the experiment being broken. After all, how can the experiment continue if the player collects all the eggs? After all, the eggs are supposedly enough of a problem that Gaster (or somebody) is sending an army of FRIENDs to hunt them down.