1. A computer system comprising: memory storing a first source data graph in a first identifier space, the first identifier space uniquely identifying items in the first source; memory storing a reconciled version of a second source data graph, the second source data graph being in a second identifier space and the reconciled version of the second source data graph being in a third identifier space, the second identifier space uniquely identifying items in the second source and the third identifier space uniquely identifying items to the computer system, wherein the first identifier space, the second identifier space, and the third identifier space differ from each other; memory storing a master evidence file that maps the first identifier space to the third identifier space and the second identifier space to the third identifier space; at least one processor; and memory storing instructions that, when executed by the at least one processor cause the system to: generate a reconciled version of the first source data graph by substituting identifiers in the first source data graph with identifiers in the third identifier space using the master evidence file, store the reconciled version of the first source data graph, and generate a combined data graph from the reconciled version of the first source data graph and the reconciled version of the second source data graph, the combined data graph being available for querying.