What are gold & silver backed by? They're just metal. They have very little functional intrinsic value aside from the propensity to be used as jewelry and over-priced HDMI cables. Big deal.
And Bitcoins aren't "backed by nothing." If you knew the technical details of the currency, every Bitcoin is backed by computing resources dedicated to validating the authenticity of the Bitcoin ledger. To me, that's some of the greatest intrinsic value possible. It ensures that no one is able to spend Bitcoins they are not entitled to.
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