The Fuffverse is a simple verse that doesn't contain much. It only contains air and nothing else. Nothing is physically stopping anyone from stepping into this verse, but there is no reason to go in, so no one is in this verse either. It is called the fuffverse because "fuff" is kind of the sound of blowing out air.
When the core of this verse gets denser, the air tends to pull anything that entered it into the core. This creates a tide-like effect, and is a natural disaster.