1. A method, in a data processing system having a processor and a memory, wherein the memory comprises instructions which are executed by the processor to cause the processor to implement an exploratory system for exploring knowledge, the method comprising: receiving, by the exploratory system, an ontology for a domain of knowledge, wherein the ontology includes a plurality of entity types and a plurality of relationships between the entity types; receiving, by the exploratory system, an input specifying a set of known facts and a semantic type of an unknown entity; identifying, by a semantic type identification component executing within the exploratory system, semantic types of the known facts; identifying, by a hypothesis generation component executing within the exploratory system, a set of hypotheses, wherein each hypothesis within the set of hypotheses comprises one or more known entities in the ontology that match the semantic type of the unknown entity; traversing, by an ontology traversal component executing within the exploratory system, a set of paths in the ontology from known facts in the set of known facts to the set of hypotheses, wherein traversing each given path within the set of paths in the ontology comprises for an unknown argument of a given relationship in the given path, correlating the unknown argument with the given relationship to generate a question using a slot filler template, submitting the question to a question answering system executing within the data processing system, and receiving at least one candidate answer having an answer confidence value from the question answering system; ranking, by a hypothesis ranking component executing within the exploratory system, the set of hypotheses based on the answer confidence value to form a ranked set of hypotheses; and presenting, by an output component executing within the exploratory system, the ranked set of hypotheses.