Time

"There is only one thing that can destroy anything and that is Time" You reply.

"Very good What is your next riddle," Stardust asks.

You think back and remember a riddle based on a wrong answer you thought of for but dismissed before and remember your favourite riddle. "Ok then my next riddle for you is one you might know and it is my favourite: I shine brightest in the dark, I am there but can not be seen, to have me cost you nothing but to be without me costs you everything."