It's literally called that: World of Nothing. Take a listen yourself:
Now that does sound scary, doesn't it?
It's one of the few instances where the title of a music piece perfectly describes its feeling. It also perfectly fits in this game too, since Super Paper Mario is one of the darkest Mario games yet (It's about a misanthropic, power-corrupted villain who wants to destroy everything in existence), which sets the tone rather aptly. To me, it has a bit of an extra layer of eeriness, as it contains a beeping heart monitor, which makes it a bit of a "dark reprise" of a much more happy and upbeat song (non-videogame) that I discovered a while back and is now among my favourite.
But it's not even the scariest music in that game; that "honour" undoubtedly belongs to River Twygz Bed, perhaps the most terrifying piece of "music" in any non-horror game ever. I've already posted that one on the previous page if you want to take a listen, but I strongly advise against doing so if you're easily scared.