1. A computer-implemented search system, comprising: an extraction component configured to extract named entities from documents having corresponding document identifiers; an indexing component configured to create and store: a document index that indexes the named entities by the document identifiers to indicate whether the named entities occur in the documents; and an inverted index that indexes the document identifiers by word identifiers of words that occur in the documents, wherein the document index and the inverted index are at least partly created in advance of receiving a keyword query from a user; a document-based search component configured to: receive the keyword query from the user, the keyword query comprising keywords; and provide the keywords to the inverted index to identify individual document identifiers corresponding to individual matching documents that match one or more of the keywords of the keyword query; a retrieval component configured to: retrieve direct matching document entities that directly match at least one of the keywords of the keyword query; and provide the individual document identifiers to the document index and retrieve co-occurring related named entities from the document index, wherein the co-occurring related named entities do not directly match the keywords of the keyword query but occur in the individual matching documents with the direct matching document entities; and a scoring component configured to: compute aggregate relevance scores for the co-occurring related named entities that occur in the individual matching documents; and return one or more of the co-occurring related named entities in response to the keyword query based on the aggregate relevance scores; and at least one processing unit configured to execute one or more of the extraction component, the indexing component, the document-based search component, the retrieval component, or the scoring component.