https://github.com/orgs/stardog-union/
Looks like Knowledge Graph and semantic reasoner are the search terms du'jour, I haven't tracked these things since OpenCyc stopped being active.
Humans may not be able to effectively trudge through the creation of trillions of little rules and facts needed for an explicit and coherent expert world model, but LLMs definitely can be used for this.
StarDog is not FOSS, that github repo is for various utils around their proprietary package in my understanding, actual engine code is not open source.