Your interpretation is the one that I think most people have, as did I for a long time. Yoda clearly states that the cave is an ‘abode of evil’ and ‘strong with the Dark Side of the Force’. If you believe Yoda, then that must be true. All I’m saying is that maybe this was another trick by Yoda to give Luke a lesson.
The whole planet is teeming with life and the Force, and Luke is tapping into areas of this Force that he doesn’t fully understand. He suddenly feels that something is wrong, he feels cold. He assumes that it’s something exterior to him since he says ‘There’s something not right’ as he looks into the darkness of the trees. Yoda immediately confirms Luke’s suspicions, but hedges when he says that the only thing there is ‘what you take with you’. This implies that there is no danger from the dark cave, but only from Luke himself. This is emphasized when he tells Luke that he will not need his weapons. Luke takes them anyway, set in his view that the darkness is exterior to himself, a foe to be defeated.
As Luke descends into the cave his fear increases. Snakes and other creatures squirm their way through the vines, adding to his unease. It becomes too dark to see clearly, and at this point he is trusting to his own feelings in the Force to guide him, feelings driven by fear and an expectation of an exterior threat. And so he sees something strange - a stone hallway cut into the muck of the swamp, draped in vines. From the hallway comes Vader and they do battle. Clearly the figure of Vader is an illusion, but the hallway is suspect as well. Who would build such a thing in the swamp, a place with no evidence of buildings or technology?
And so comes my point - why should this place be different from any other place on Dagobah? All places on the planet have life and death, fear and peace in similar amounts. The great processes of life do not change simply because they happen in a creepy cave, so to believe Yoda we must accept that the cave has some Dark Side associated with it from sources unknown. Since this source is never revealed then it’s anyone’s guess if Yoda is telling the truth or merely playing on Luke’s own feelings.
My interpretation is that the real reason this cave is strong with the Dark Side is because Luke is there, and the Dark Side has a hold on him. The cave is just a cave, drenched in the living Force but not necessarily good or bad. He turns the energy in the cave to the Dark Side with his expectation and fear, and it responds in kind. He could have had the same experience in the woods or swimming in the swamp if his expectation was similarly primed and his bearings taken away. Keep in mind that during a similar training exercise he has a dramatic vision without being in a place that is established to be strong with the light or Dark side. A vision can happen anywhere to Luke as he opens himself to the power of the Force; it is only the suggestion of Yoda which directs the course of the Dark Side lesson.