If something's unacknowledged, it hasn't been recognized or made public, like your unacknowledged fear of the dark — it exists, but you've never admitted or even spoken about it.
Unacknowledged things have been ignored or disregarded. And it's the same when a person is unacknowledged: she deserves to be recognized for some accomplishment or quality, but she hasn't been praised for it. A brilliant artist who isn't able to sell many paintings might be an unacknowledged genius, for example. This word comes from acknowledge, a combination of the Middle English aknow, "show one's knowledge," and knowlechen, "admit."