My understanding is that it is not in the Bible. However it features strongly in the Qu'ran where it appears to be explicitly created by Allah (the Merciful?) to forever punish those who fail to exercise their free will to worship Him.
There seem to be three possibilities for the origin.
1. The Catholics get it from the Qu'ran.
2. Early Catholics created it and Mohammed liked the idea.
3. Those who wrote the Bible neglected to include it when transcribing the rest of the old oral myths the books are based on.
The idea of Hell as eternal punishment did not exist within Judaism, but arose with Christianity, and was subsequently adopted by Islam, so number 2 is the right answer. It keeps the faithful fearful.
//Lots of things exist in the mind but that doesn't mean they're real.//
But they can seem as real and threatening to those who don't know what to believe and why. The dire prospect of hell is limited only by the imagination of those who are willing to accept in the absence of certain knowledge the beliefs that others are all to willing to promote in the hope of capitalising on their ignorance.
Keyplus, I do have worries but not about god, My worries are about people like you who try to propagate ignorance ,fear and superstition and live in a hell of their own makng.
Keyplus simply cannot comprehend that other people are not intimidated by the horrors that religion promotes. That, sadly, is the result of life-long indoctrination.
Mibs, //But they can seem as real and threatening to those who don't know what to believe and why.//
True, but the ‘why?’ is more important than the ‘what’. If you want to rationalise your beliefs, rather than examine what you believe, you must examine your reasons for believing. First and foremost you have to ask the all-important question - ‘why?’