The main asteroid belt was at one time thought to be the debris of either a planet that got destroyed in a collision, or the potential material that never quite formed as a planet in that location. Nowadays it's generally thought to be material of various origin that has been shepherded into a ring of debris by Jupiter and the Sun.
Knew that. Personally I think it's too close to Jupiter and too confined for that to be true.
I like, "We don't know yet." It's true of too many 'scientific conclusions' in my book.