If you know me then I am nothing, but if you don't, then I am something What am I?

“A secret”

While a secret can be something unknown to someone, if you know a secret, it doesn't necessarily become 'nothing'. It still holds value and meaning.

“An illusion”

An illusion may seem like something when perceived, but it doesn't fit the criteria of becoming 'nothing' once known; rather, it is recognized as false.

“A lie”

A lie remains something regardless of whether you know it or not. Knowing a lie does not diminish its existence or impact.