OK, so I have the 2009 edition of the Marvel Encyclopedia, which has 8 pages devoted to The Hulk. Apparently he had “every atom of his body exposed to gamma radiation” from a gamma-bomb he was working on. It’s been revealed that Banner also has multiple personality disorder that contributes to his various Hulk forms.

I thought I remember from decades ago that his bulk and healing energy comes and goes from a transdimensional space, but I can’t seem to find that in the article.

However, I really like this StackExchange answer, which posits that he is an energy-processing mutant that taps into the Power Cosmic to fuel his incredible strength. https://scifi.stackexchange.com/a/84410