The following items are discussed by various theoretical physicists as if they have been proven to be real entities in the physical cosmos. Actually none has ever been observed and most of these ad hoc unicorns probably never will be observed because they are pure fiction.
Confined quarks (conveniently unobservable by theoretical fiat)
Inflaton field (supposedly powers inflation)
Higgs boson (too unstable to be directly observed; must be inferred via decay products)
“But”, you say, “these are crucial components of our standard liturgy for theoretical physics.” Exactly so! All these mythical epicycles may in fact be making a mockery of science, which is based on predictions AND empirical testing.