When Dr Bruce Banner becomes The Incredible Hulk he increases in height and presumably weight. My question is - how does that mass increase occur? I'm looking (obviously) for an in-universe explanation.
The Marvel wiki entry for The Incredible Hulk states in the "Powers" section:
The gamma radiation that mutated the Hulk's body fortified his cellular structure and added, from some as yet unknown (presumably extradimensional) source, several hundred pounds of bone, muscle, and other tissue to his body.
As noted below by oli, this source has recently been clearly and unambiguously identified in the pages of "Immortal Hulk".
I'd like to point out that I joined this site and raised my score, specifically to answer this question, since it was protected... and now here I am.
First, I need to respond to the answers already here...
The Hulk does not absorb ambient gamma radiation and use it to cause mutation. There is simply not enough ambient energy to fuel the mass he gains. Additionally, note that, on more than one occasion (for example, the poisoning of his wife and his transformation to World Breaker), he is actually EMITTING, not ABSORBING gamma radiation. He's generating it. Note that he is also utterly immune to the effects of gamma radiation, and always has been, so he CAN actually absorb radiation. It just doesn't fuel his mass gain.
The Hulk DOES have phenomenal cell growth. It just is not because of gamma radiation. Such a transformation would have him grow some, and then begin tearing himself apart to get the proteins he would need to continue growing. It's just not feasible.
The Hulk does NOT generate the antimatter that would normally be generated when creating gamma radiation. This would cause massive cellular destruction, along with the damage the radiation itself would do. In fact all of the above answers would have the Hulk destroying as much as he gains, and thus transforming every few minutes. It would also, incidentally, cause massive disturbances in power and communications around him, which has never been the case.
I generally try not to repeat my answers, but since I came here for this one, I will restate what I have restated in other answers, to support my case for my opinion.
- Bruce Banner's father believed he was a mutant. (Incredible Hulk 312).
- The Hulk can see astral projections (as he has seen Dr. Strange in his astral form). This implies that there is a telepathic component to this transformation, as telepaths derive some of their power from the astral plane (Xavier, for example, can astral project).
- The Hulk cannot be mind controlled for long, by anyone, by any means. Several have tried by magical means, including the deity Loki, and by telepathic means. None succeed for long. Considering his purportedly low intelligence (generally speaking), controlling him should be easy... yet it is not. This again supports that there is some telepathic or astral component to his transformation beyond his rage.
- The Hulk has been reported by Marvel comics to gain up to 1250 lbs in mass. The amount of energy required to accomplish this (510291450000000000000000000 ergs) would not only drain the power of the entire planet, but would do so for hundreds of years.
- The Hulk can deliver impacts in the order of more than several quadrillion tons of force. Yet, he never blasts himself off the planet by punching the ground (which is exactly what would happen. He'd reach escape velocity in an instant.)
- The Hulk cannot be held in any alternate dimension, as he has broken out several times when placed there by Dr. Strange and others, including pocket dimensions.
- The Hulk's pants never break off. And they remain when he changes back, still fitting Banner.
I propose the following:
Bruce Banner has some form (actually, a relatively mild form) of multiple personality disorder. This has created a "mild" persona that is filled with self loathing (at his weakness and inability to protect himself and his mother) and is always on the defensive, fearing everything, and wanting to crush everything.
The trauma of meeting death at the hands of his own creation finally fragmented the mind of Bruce Banner, his creation being the gamma bomb itself. And he hated. With all his being.
The Astral Plane, the dimension of psychics and sorcerors, is the source of the POWER COSMIC (as stated by the Silver Surfer). It is also where EVERYONE'S mind "exists" manifested. This is where the Hulk "lives" when he is not in use.
Bruce Banner is a MUTANT who metabolizes radiation - which makes him immune to ALL high energy radiation - and uses it to catalyze a connection to the ASTRAL PLANE, manifesting the Hulk, an astral projection, as REAL. The HULK is actually the Power Cosmic ITSELF, manifest INSIDE the cells of Bruce Banner (a completely unique ability).
This is why the Hulk cannot be contained in any pocket dimension, ESPECIALLY not the astral plane. This is why the Hulk can gain mass and strength as he sees fit. This is why the Hulk cannot be controlled by any psychic measure. This is why the Hulk can TRANSMUTE any part of his cells or organs to adapt to his environment (the Power Cosmic is primarily the power to transmute energy/matter). This is why the Hulk seems to not follow physics (he's not hitting with his actual hands, he's beating people with PURE FORCE ENERGY, though HE DOESN'T KNOW IT because he BELIEVES IN HIMSELF). This is why his pants never tear off! (They are actually figments of his imagination, and if Banner believed he'd be fully dressed when he came out of the transformation, he would be.) And this is why, not only can the Hulk change personalities, as Bruce Banner himself comes to terms with his feelings about the Hulk (becoming intelligent, or weaker, or stronger, or whatever)... but why BRUCE and HULK can be SEPARATED! Because, once created, the Hulk believes in himself, and is actually a separate entity from Bruce Banner himself, though connected.
This is why when Bruce Banner has his gamma radiation drained, he cannot catalyze the reaction to "transform". And why, when the Hulk is strong enough, he emits radiation, self-catalyzing the reaction himself.
What do you think?
(and yes, if Bruce and the Hulk were separated and Bruce was killed, the Hulk would also die. This is why the Hulk cannot actually physically kill Bruce Banner, despite wanting to. He can, however, mentally kill him and stay the Hulk permanently.)
I supose that the accident that created hulk has mutated Bruce Banner's cells making them unstable. When he transforms the radiation in his body produce a phenomenal cell growth, those new cells are also mutant cells, way harder and stronger than any human cell, getting a curious green tone on skin cells due it's source.
When hulk becames calm and reverts to Banner's form the gamma radiation fades, so the new cells can't continue it's existence and die. However, those mutated cells don't die in a normal way, they just disintegrate, returning the cellular structure of Bruce Banner to it's previous state.
I know that's not canonical, but I like how it sounds.