Under the doctrine of Erie Railroad Co. v. Tompkins (1938), there is no general federal common law. Although federal courts can create federal common law in the form of case law, such law must be linked one way or another to the interpretation of a particular federal constitutional provision, statute, or regulation (which in turn was enacted as part of the Constitution or after). Federal courts lack the plenary power possessed by state courts to simply make up law, which the latter are able to do in the absence of constitutional or statutory provisions replacing the common law. Only in a few narrow limited areas, like maritime law, has the Constitution expressly authorized the continuation of English common law at the federal level (meaning that in those areas federal courts can continue to make law as they see fit, subject to the limitations of stare decisis).
Try to answer this question if possible (otherwise reply "unanswerable"): When are states allowed to use plenary power?
absence of constitutional or statutory provisions replacing the common law