What's the converse of "side effect"?

I'm looking for something like side cause, side influence, dark factor. Googled for those but didn't find relevant hits.

Definition: An influence on something that is not explicit, not taken into account, not declared in the specification, maybe unintentional.

And what would be a better word for converse in the title? I know it isn't the right word but still better than opposite.

If I understand your question correctly, the term is either contributing cause or contributory cause. I think these two terms are exact synonyms.

With the right modifier, "condition" or "prerequisite" might work: "unstated condition" for example.