It will always be a theory, because it will always be a model. It is based on assumptions and understandings, and these are never 100% perfect. What most people do not know is that many other theories have been tried, including one where the energy density remains constant (and new energy is constantly being created as space expands).
All of these other theories have been shown to be wrong. The one where the energy density remains constant was called Steady State, and, for roughly 20 years, was considered better than Big Bang. Unfortunately, under that model, the Cosmic Microwave Background radiation should not even exist, and primordial gas should only contain hydrogen (no helium).
Steady State suddenly lost its luster in 1964. It was finally completely abandoned recently, despite one last attempt to "modernize" it in 1993.
The name "Big Bang" was NOT the name of the theory. The name was given as a joke, by the guy who was pushing the other theory (Steady State). The name came from an earlier misconception that the theory describes an initial explosion of very dense matter. We now know that there was no matter at the earliest possible moment that we can study (the moment called the Planck Time).
However, at the Planck Time, the initial energy already existed and space was already expanding. Therefore, the Big Bang theory is NOT a theory about the creation of the universe. If the universe was "created", then that happened "before" the Planck Time.
---
IF the Big Bang theory turns out to be false, we are still stuck with this list of facts:
--space is expanding (this is an observed fact)
--the energy density is decreasing; this can now be measured by looking at distance objects (in astronomy, when we look at things far away, we are looking into the past)
--the universe was hotter in the past than it is now
--there was a time (when the Cosmic Microwave Background radiation was emitted) when the universe was over one thousand times hotter than it is now