@whit it's backed by work of miners. and therefore electricity.
I know it's not the same as gold, but it is "backed" by something, *the miners*. the miners mine because they get paid in BTC to do so.
Mining is a misnomer, miners try to sign the current ledger with new additions by causing a SHA256(SHA256(ledger)) to result in a specific amount of leading zeros. The number of 0s is called the "difficulty".