Try to guess just what I am. You'll find me in the middle.
I'll give you clues so you can guess the subject of this riddle.
I am truly an ingredient in a thing you love to eat,
And yet you do not eat me, for I'm neither plant nor meat.

I do not add a flavor. I am not checked for weight.
And yet I am in tasty treats, as they lie upon your plate.
If bakers do not use me, then something else, usually,
Is used to occupy that space that isn't used by me.

Although I'm quite important, no one, for goodness sake,
Checks to make sure that there's enough before they start to bake.
The origins of my first use cause men to search and ponder
For reasons why I'm used at all. "Why is it there?" they wonder.

There are a lot of theories as to why I came to be.
The fact that you expect me there is reason enough for me.
So eat me and enjoy me, though in fact you can do neither.
I add nothing much at all, and I don't add calories, either.