The white paper of bitcoin had already mentioned about the halving and how it will take place every 4 years and decrease the block reward by half of the current block reward. It's definitely not just a theory. I think you should read the bitcoin white paper once.
It has great knowledge about bitcoin and it's technical functionality on the blockchain.
Yup, it's really easy to realize the OP hasn't read the WP. It's not just great knowledge but it explains the ins and outs of the mechanics, for example the point of the halvening is introducing some extra escarcity, so that is not just the 21m cap that drives price up, also make s it so that it takes longer to reach that 21m