I don't think you understand what you're even asking; you're just parroting what you've heard other objectionists say. The importance of being "backed" by something is so that it is anchored to something that we *know* has value, based on its inherent utility and finite supply, thus ensuring that the currency always holds a consistent value and can be relied upon as an agreed value exchange mechanism.

Bitcoin doesn't quite follow the traditional currency story though, because its utility and finite supply is built in, thus obviating the need for it to be backed by some other commodity of value. It is in this way that we can say that it is backed by mathematics, cryptography and the internet, which are the foundation of its usefulness and limited supply.