The reality of unobservables
The reality of unobservables is closely related to epistomolgy, a term that generally means how one views the world, or their "theory of knowledge". Some argue that if a thing cannot be observed, it cannot be verified - hence the saying "if a tree falls in the forest, and no one is around to hear it, does it make a sound?". One theory is yes - we have observed other trees fall and they made sounds, so that one did too. Others would say that one has no way of knowing if it made a sound or not because it was not observed.
The reality of unobservables, therefore, takes on a special and very important meaning in the philosophy of science. Many of the recent innovations in theoretical, high energy physics is done through induction and indirect evidence - no one has seen quark or other subatomic particle; they are unobservable. But scientists believe in the "reality" of the subatomic particles because of the evidence they have amassed to prove their theory.