Formally known as a "Hoff's Gravitational Inversion" (H.G.I), a "BubbleVerse" is a gravitational anomaly, a self-contained and highly unstable replica of a small geographical area.
According to the F.B.P dossier on such anomalies;
...the most commonly used "real world" analogy is a change in air pressure that causes a smaller soap bubble to form on top of a larger one...
...with our dimension acting as the larger bubble, an H.G.I is formed when a weakness in the membrane allows gravity to drag a localized section of our dimension through to the other side of the 'brane...
...a dulplicate of the affected area is produced on the other side, and contained in it's own self-contained bubble, hence, BubbleVerse.Source: F.B.P #2
Because of the nature of the BubbleVerse, it is similar to a parallel universe, Special Agent Cicero Deluca describes it as "a fun-house mirror version of our world" and as it "moves towards collapse, it'll only get worse." An example of this weirdness is that the people whom inhabit the H.G.I are merely molecularly unstable and very malleable. As such, due to a construction accident, two civilians are merged together after one falls on top of the other, only to melt away afterwards.