The Persian Puzzle

I am in a box labeled “Atoms of Peaceful Progress” –

the lid is askew, offering a dim, limited, inside view.

Though the material of the box seems quite opaque,

the owner insists that it’s absolutely transparent.

There’s no consensus on the box’s degree of opacity,

let alone the precise nature of its mysterious contents.

I am just one thing, amongst many things in the box,

but I am that which most crucially needs knowing.

My exposure would negate or confirm all suspicion,

yet I defy identification – even by skillful illumination.

What am i?