image from artrenewal.org
Mystery is quality a painting can have. Instead of everything being delineated precisely some passages are lost in darkness or simplified into tones that give the viewer no clear idea of what is represented in that passage. The Innesss above is a good example of this . The foreground and the shadowed areas of the trees are full of mystery.
Paintings that explain every detail can be brittle, or nervous looking. They have a primitive look. Some of the Hudson River School painters had this fault. The next generation, the tonalist replaced them rapidly and one of the reasons why was the quality called mystery.
Mystery lets the viewer assemble the picture a little for themselves. But it also serves another purpose. The passages that are full of detail are counterbalanced against those which are not. Covering an entire canvas with writhing assertive detail gives a hyperconcentrated and febrile look. A painting entirely covered in tiny details causes the viewers eye to skitter madly about its surface, trying to apprehend all that is there, and never finding relief or a place to rest. We don't see that way either, we are aware of the details of an area on which our gaze is fixed, but all around that in our peripheral vision we are aware only of the large shapes and colors.
Naive painters, and folk artists generally don't use mystery and that is one of the ways we recognize their work. Photorealism can , despite its seeming modernism have this primitive fault too. The artist has failed to be selective. This is one of the dangers of working from photography, the camera sees everything before it with critical selection. Irrelevant details are presented with the same "laundry list" exactitude as the most important things on the canvas. Paintings like this look mindless, and are hard to read and enjoy.
There is a famous quote from Miles Davis "Its not what you play, but what you don't play. A frequent criticism of some musicians is that they play too many notes. It is like a dinner guest who tells extended boring stories full of useless detail and long wandering asides, that bore everyone at the table and never come to their point.
When you paint, try to leave some areas, particularly those which need to be there but aren't really part of your story, deliberately vague. Nature needs to be edited by the artist rather than copied.
Mystery is had by simplification. Leaving shadow areas simple and painting only representative detail rather than a catalog of every little little thing helps make a painting more poetic and less journalistic. Try to learn how to say just enough to make your picture work and no more. Contrast detailed passages with simpler passages, and your work will have more sophistication and poetry.