It's not that hard is it? that cloud layer has clouds in it, clouds cast shadows. no cloud means no shadow to cast. unshadowed area's of any object are receiving more light, thus looking brighter. Since the earth has an atmosphere the sky counts as an object too. Compare it to a volume atmosphere in bryce for instance. There doesn't need to be a cloud 'behind the stream of light' to see the extra brightness. Imagine you fly into space with a rocket. You look out of the window and the space u see is black. Space is black because it's a vacuum, pure vacuum has nothing in it to receive light, thus nothing to change the lights direction towards our eyes. If you're in space, in a vacuum, only the light that travels directly in our direction is seen (like the stars) around the stars is pure blackness. Now imagine your rocket that took u to space before is going back to earth and land you safely on the ground again. The lower you get in the atmosphere the brighter the sky looks. Why does it do so? Because it's not a vaccuum anymore. Particles of whatever, gasses, waterdrops, ('stuff' as PJF called it) they all can in one way or another change the direction in which the light travels, thus allowing some of it to bounce in the direction of your eyes which will receive the light, even if you're not looking directly at the light source. Just think of it this way, the more light passes through a certain area in the sky, the more light gets to be bounced into the direction of your eyes, and thus the brighter the area will look. Just like the streaming light rays in your brycean volume atmosphere scenes ;-)

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