The Unimaginable is a verse that is unimaginable in every possible way. It's shape constantly changes at SUCH a fast rate that even Gerald, Dunkey Kong, or other Mary Sue type gods have a very hard time comprehending its true size, even with The Core of Everything. There are no pictures of it due to if you took a picture of it, it probably already grew TO you, and ONLY doesn't apply if you're The Camera Man, resulting in your immediate demise. Since all factors about it are constantly changing at an unknown rate, it's very unstable.
This verse is SO unstable that it actually duplicates itself infinite times every single Voidsecond, making it even more unstable than the infinith layer of the Ultimate Continuum. The ONLY reason it hasn't been completely annihilated from its own energy is due to A Bond keeping the Unimaginables in place as still as they can be. It's theorized that if The Bond wasn't holding them together, the unimaginable would have immediately "poof"ed from existence by its own energy and it would have caused one of the most destructive events... ever.
How it's bigger than the Indescribable
Basically, the unimaginable is SO unstable that sometimes the bonds can't separate the unimaginables from eachother, resulting in the unimaginables releasing a GIANT wave of energy infinitely larger than the unimaginables itself, and it keeps doing this at completely random times. It's the single most powerful event in all of any verse's history, and since that energy still counts as the unimaginable due to some strange property, it can grow infinitely bigger and duplicate at bigger sizes, etc. It currently has no known bounds other than always being a part of a larger and larger Bond.