It's backed by the tremendous amount of electricity required to run the transaction processing network (ie. "mining"). Remember this is the world's largest distributed supercomputer.
BTC is a proxy for electricity, that's why mining equipment ended up at ASICs so quickly- in mining, less W = more $$. By a lot.
The USD is currently backed by absolutely nothing. There is no way we could print the $2000 trillion dollars[1] of derivatives now in existence. You couldn't cash out all of it or even too much of it.
The USD is "fake". And high finance is not ignorant to this fact, at all.
[1] Or more, no one actually knows. Goldman Sachs would have the best idea of this.