We are trying to index punctuation as much as possible, e.g. https://duckduckgo.com/?q=%28
However, most of the APIs we use to help generate results don't, and so it can be a bit sparse in that regard. That's why in the example search you don't see any traditional looking results below our instant answer box.
Our instant answer index does index full punctuation. As it is built out further, more and more searches with punctuation should return relevant results.