Literally Metaphoric

This is literally metaphoric in it’s poetic essence,
The lack of its existence is the thing that proves its presence,
It’s the missing link on the weakened chain that holds it all together,
It weighs down the pack because you see it’s lighter than a feather,
You’ll never cross it because it does only what it chooses,
Ironically the path you make is the same one that it uses,
It seems as though it’s never there when the sun shines bright,
And it only comes out into view when it’s too dark for your sight,
It’s like the shadow cast by the bird that passed over the bright blue sky,
It moves along never right nor wrong as the bird continues to fly,
Just like the light that hits the water at the exact moment of dawn,
This poem will end but its presence will forever go on.