Can I have depression without a real cause?

Earlier in my life, I was depressed for a periods of 7 years. But, because the word depression is used so loosely, my parents, siblings, and even peers didn't believe I was depressed. In short, I was unofficially depressed for all that time with no one willing to help me and was never diagnosed. About a year ago, I am not sure how, but my depression seemed to weaken for a little over a year. Since I didn't want my depression to come back, I decided to do some research on how it is caused. Most of my results stated depression is caused from: rejection, loss of relative or loved one, failure in academics, stress, etc... Despite finding all these causing, my situation didn't fit any of those causes. Then, a couple months back, my depression came back, and is still not diagnosed because no proof of a cause was found. I have been wondering and wondering for the past few years but still have no answer. So, is it possible, if diagnosed, to have depression without a real main cause and yet still have its basic symptoms: yes or no? And please explain your answer.