system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel] What IS the Hulk?? Is he from another dimension? Where does he go when he transforms back to Bruce? Does everyone have their own hulk monster inside them?
gpt
The Hulk is a human mutate who has gained power from an exposure to gamma radiation.  Normally exposure to such radiation would be a sure death sentence, but for some reason Bruce Banner's body adapted the energy from that radiation and allowed him to survive.  He is fueled by rage, which originated in an abusive, or at the very least neglectful, childhood.  It's implied that he was psychologically unbalanced even before his exposure to radiation.

The Hulk's personality is part of Banner's subconscious, so it is always there, even when Bruce Banner is in control.