A ceiling made of darkness.

Its existence is not the reason we can see it. The focus of our eyes produces the perception of existence which we see. 

In other words, you are not seeing the ceiling of darkness. The continuous focus of the viewer creates the ceiling of darkness in the cognitive world, and that is what you are seeing.

The recognition of existence is created from the viewer’s ability to continuously maintain their focus.

The ceiling of darkness exists in the viewer's cognitive world. We call artworks that exist in the cognitive world, but not in the physical world, Cognitive Sculptures. Existence in the cognitive world makes the artwork come into existence.