I think "Expansion Theory" was the more correct term. If I remember correctly, "The Big Bang" was a term coined by someone who was making fun of the idea. Only, it caught on to general use.
Also, I don't think "the big bang" or "expansion theory" is really meant to explain a first cause. Rather, it is a theory meant to explain how the universe arrived at its present expanding condition. Since the universe appears to be expanding (and cooling) at an accelerating rate, it can be logically inferred that you can deduce backwards to a single point condition (an infinitely hot and dense singularity). What caused it to expand in the first place is not really part of the expansion theory. That's my understanding, at any rate.
To me, there also seems to be a relationship between this theory and the idea in quantum physics of energy/matter at the theoretical Planck Level. (Where energy/matter cannot be divided into smaller parts and is infinite and everywhere at the same time.)
Biblically, the theory could be understood to coincide with a statement in Isaiah that God is 'spreading out the heavens.' (Isa 40:22b; Some translations take "the heavens" to refer to "the sky.")
It is interesting that "The Big Bang" is usually (in my experience anyways) dismissed as non-sense in Kingdom Hall discussions. But ironically, I think the Society actually takes a wait-and-see attitude towards it.