When you know me, I am nothing when you don't know me, I am something. What am I?

Question

I'm something. I'm nothing.
I am short. I am tall.
When you fall at your sport, then I stumble and fall.
I have never been seen beneath a new moon.
I thrive in the evening but vanish at noon.
I am lighter than air, I weigh less than a breath;
darkness destroys me, and light is my death.
What am i?

Answer 1

The answer is secret. If know any secret then it's not a secret any more.