UFO or big airshipIn my opinion UFO's have existed for several decades already on this earth. But they are human-made. I can imagine that they have come up with a cloaking device or they can hide in plain site just like in a cloud. You could think about a UFO that can make it's own clouds around it's ship. So we couldn't see it. This is one option. The second option is what we see here is a big airship (for instance like the Hindenburg). From 1938 (accident Hindenburg) till now they have perfected the technology of this flying airship and it might be also using cloaking or cloud-making technology. So what about that shadow. An airship needs to use rudders on each side of the airship for steering. If that rudder goes in front of the local sun, then you would see such a shadow line within the clouds.
Edit 1 September 2017: Controversially it might be God watching from above the clouds.
Source (1): http://chemed.chem.purdue.edu/demos/main_pages/6.5.html