Joined: Mar. 2005
Biology and Evolution theory involves the development of life and relationships among species. The development of planets, stars, or the whole universe belong to whole other subjects like astronomy, cosmology, and/or astrophysics.
As for string theory, last I heard it was really a hypothesis (i.e., not yet directly supported by evidence) rather than a theory that is supported by enough evidence to give it widespread acceptance. (Then of course there's the question of which string theory, unless they succeed in consolodating them into one.)
Besides which, there will always be something that can be only observed and described by science, rather than being explained in terms of something more basic. That's because explaining something sort of requires that we know of something more basic than the thing being explained. Right now the most basic thing (described in widely accepted theory) seems to be quantum theory (plus relativity theory), but if string theory (or one of them) is successful it would become the described (but not really explained) basis for the rest.