You've just indicated the reasons it's called invisible money. Of course it has no physical existence, nor has no common document. It's just your wallet and the number of your coins. That's why it's easily decentered and can be kept anonymous.
This got a point. We cannot see it physically because it ks the digital currency, it is in virtual world that can be used to interact with the physical goods and services. If it does not have any value why is it then more people rather more and more people are using it. This probably got a point.