"How did you get here? And how do you get out of those boxes? It is too big to even comprehend. Everything is unknown and impossible."...
If you escape The True Boxium then you will end up here, this black to white oval of impossibility. Everything here is impossible. Those boxes were just containing some of the impossibility. If you manage to end up here, it will be very broken and unknown. So unknown that you don't want to end up here.
The Impossibleverse is a verse that contains The True Boxium or even nothing or... impossibility. Everything here will disappear and appear again multiple times and can even permanently disappear. If you end up here, you will see things fine until you permanently disappear. Some people manage to get here and permanently disappear for the rest of eternity but the identity is still kept.
Entrance[]
If you want to come, then you need to noclip to the walls of The True Boxium. But instead of a normal noclip like you just need to run. You need to travel faster than a tachyon or light to noclip out. You'll not get into The Backrooms, instead into this verse.
Exit[]
If you want to exit, you'll also need to not exit because TH3V01D is NO ESCAPE... If you want to... Then you need to noclip into the walls faster than a tachyon or light. Then you'll end up in TH3V01D.
"I've kept a secret from you but I will never tell what it is. Probably you can guess it but you won't try. The Impossibleverse only can show it, not me." "Well it is something about the Impossibleverse. Hidden there. An impossible secret. I won't tell you, only this verse can. ONLY the Impossibleverse can. Only that one. If you found it, you're lucky."...
The secret was that you can go back to The True Boxium from the Impossibleverse. It's probably not easy. Noclip into one of the The True Boxium's walls but faster than the speed of light or tachyons...