When you don't know what I am then I am something but when you know what I am then I am nothing? 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

A mystery, danger or a riddle.