Personally, I find the idea of a "pocket universe" something of an overkill. My own explanation (posted a while ago in a Hulk thread) involves just one universe: ours.
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"The Hulk's power is ultimately grounded in psionics.
The gamma rays changed those parts of Banner's brain which affected aggressive feelings, behavior and strength--the "fight" part of the fight/flight response. They also skewed his brain's ability to sense the universal psionic field (yknow, the one so many other heroes tap into). When Bruce gets pissed, that ability kicks in, creating an intense, but localized conversion of energy into matter.
The energy comes from the quantum vacuum, a frothing of virtual particles. These ghost particles are empowered psionically. This is where all the Hulk's mass comes from: matter out of the vacuum of space, being adapted by his Fight response. It also provides the virtually unlimited wellspring of energy from him to draw on, as he gets angrier and angrier.
This also explains why his pants are nigh-indestructible. The Hulk may be monstrous but he's still human. At a deeply unconscious level, Bruce's modesty is still fractionally intact. As the pants are damaged, they are instantly regenerated (as best as possible), just like the Hulk's body regenerates (only living tissue takes longer cuz it's more complex).
This further explains why the Hulk can see astral (psionic) forms."