No, you're not gonna learn actual theory and science and philosophy, that is boring. But what I am gonna theorize is Danny Phantom. This theory (fact) is that the ghosts were once real people. Okay, so that is not a theory but next... The next part is me theorizing how they died. There may be some out there but Imma list off the ghosts I can think of and give a theory on how they died, the ones that doesn't have a story yet.
The Lunch Lady/The Box lunch
The Box Ghost
Technus
Skulker
Ember
Bullet
Walker
Youngblood
Termintara
Penelope Spectra
These are just some theories, but not all ghosts. There are some that already have back stories, just the one's that don't.