Judging by the angle of the shadows and the color of what light is visible in your screenshot, it looks to be either morning or late afternoon, and yes you are seeing a large shadow of a mountain, you can clearly see bright areas on the ground in places where the light is coming over whatever is blocking it.
What others have said is true though, different monitors will have vastly different contrast capabilities and gamma curves which will change how you see an image. On my monitor everything in that screenshot is perfectly visible, although a little dim due to the large shadow.
So the title of this post isn't really accurate, as your screenshot was not taken in broad daylight. The sky is the last thing to get dark in reality, as light from the sun refracts through the atmosphere above you lighting up the sky even after it has gone down past the horizon.
If you didn't know, you can type /time in game to check the time.