It's behind you

First thought, and one that has bothered me for a while now, is why can't you see behind you?
Seems obvious right? It's cos our eyes are at the front.
But my point is where does your vision stop? And what does the brain fill it with?
Imagine your head is a sphere, like an orange. If you cut it in half, roughly from ear to ear and over the top of your skull, one half, with your eyes in it represents all you can see from one edge of your peripheral vision to the other. The other half is the back of your head. As far as your vision is concerned it doesn't exist.
But concentrate for a minute, stare at the screen but think about what is at the edges of your vision -for me it's the book case one side and the wall the other. Now I know what is beyond because it's my room, but there is no black line where the edge is, there is no big black void where I can't see. On the other hand the image of the bookcase is 180 degrees from that of the wall. They don't join up.
So what is my brain doing? Ignoring it all? It certainly isn't filling it up with dark space. I am completely unaware of what is behind me until I turn to see it.
Do babies have this void? Or is it something our brains just learn to ignore so that by the time we are walking it is so invisible we can't even see it if we focus hard?

What is behind you?