Normally, “just a theory” can be wrongfully mistaken as indicating the uncertain nature of “theory,” however, in theoretical physics, “just a theory” actually carries more weight as to show the achievability of a concept.
In either case, a “theory” has been proven in one way or another, but may not have reached consensus status in the scientific community thereof.
Like how geometrically, using general relativity, one can come up with a situation in which a worm hole can be created, but in nature, the things necessary for that geometry to be achieved does not exists as far as we know.