1. A method for operating a search system for searching a search space comprised of domains each representing a collection of related documents, comprising: establishing a domain characterization model (DCM) comprising a DCM index containing keywords and, for each keyword, an identification of a domain where the keyword is found; and in response to an original search query, forming at least one enhanced search query in accordance with the DCM for searching a plurality of domains, said at least one enhanced search query being formed for targeting a particular domain and comprising the original search query and at least one keyword associated with the particular domain.