[eru]

> Is it even possible that a complete simulation of the rules of this universe (let alone the content of it) could exist in this universe?

Yes, it is possible! See how a Quine is constructed.

https://en.wikipedia.org/wiki/Quine_(computing)

As an analogy: you can have a zip archive that expands to itself. Or even more so: to itself (the simulation of itself in itself) plus something arbitrary (standing in for the rest of the universe).

[CorrectHorseBat]

There's also information in the compiler and the zip binary, without that I'm pretty sure it's not possible.

[eru]

You can stick a full copy of the compiler and the zip binary into that archive, if you want to.

The 'intron' can be arbitrarily large.

You can add the complete schematics for your CPU. And the standard model of particle physics.