Whenever I read "It's estimated that there are around X ys in the observable universe." How in the hell could you figure that out, when there are a lot of thi

Whenever I read "It's estimated that there are around X ys in the observable universe." How in the hell could you figure that out, when there are a lot of things in physics and astronomy that we do not yet know how works, nor how they are composed. It is the ultimate "estimate how long it will take to implement this computer system based on this napkin drawing I made in 3 minutes ago".