Luke’s test in the Dark Side cave on Dagobah was purely a fight with his inner darkness. Through his training, Luke was sensing more and more his own dark potential which Yoda redirected by telling him that the darkness existed within an entirely normal cave. This allowed Luke to exteriorize an interior feeling and bring it to light.
Ooo, I love this idea. I prefer it being a total psychological thing rather than there being a “Force nexus” tied to a physical location.