Beyond your comprehension and beyond this world, trapped within unseen walls.
Beyond your comprehension - Ness has no idea what that picture is supposed to be of, and it makes him tired just thinking about it.... snooooreee...... no math.... math.. no.... *droooooolll*
and beyond this world, - It's in Moonside (you know, the place with all the cool neon lights that the Mani Mani statue created), which is an INCREDIBLY out of this world place. Remember kids! Say YES to veggies, and NO to dessert!
trapped within unseen walls. - Moonside is divided up into all these invisible barriers. If an Abstract Art sees you while you're in a section other than the one its in, it will furiously try to get to you, but will not be able to travel through the barrier. Heh heh, it was fun to tease em'.
It seems the popular guess this time was Moonside. Everyone who guessed that was DARN close, but it doesn't quite work. It does seem to be beyond your comprehension and beyond this world, but it isn't really trapped within unseen walls, as the unseen walls are a part of Moonside itself. Also, some people guessed the invisible guy with the Unibrow and gold tooth. Well, the first two apply to him as well, but he isn't really trapped, as has the ability to follow you around.