How to understand what bitcoin is "backed by".

When government currency was backed by gold the gold did one thing, it enforced a set of rules that made sure no more currency units could be produced than there is gold to back it. Essentially gold backing stops over issuance of currency units preventing inflation. The gold isnt the thing giving currency its value because there is no guarantee a piece of gold can ever be redeemed by an individual, nor does the price that gold is set at in order to back a government currency matter, all that matters is that the gold backing prevents inflation of the currency supply. Gold backing gave a currency its value because it simply forced it to remain scarce and that scarcity is what gave it value. All something needs to be valuable is utility and scarcity.

Bitcoins scarcity is enforced through immutable code instead of through backing by a physical scarce commodity, and that immutable code is immutable because of the node operators and miners who run the network.

This means bitcoins scarcity is enforced by computing power, therefore the commodity backing bitcoin is pure energy because it is energy that is enforcing bitcoins scarcity.

Using physical gold to enforce scarcity is a rule set enforced by a physical asset. Using the worlds most powerful decentrazied computing network to enforce scarcity is a rule set enforced by energy and immutable code.

Gold is a physical object that enforces a set of rules that maintain scarcity. Bitcoins decentralised computing network is an object that enforces a set of rules that maintain scarcity.

Both bitcoin and gold dont need external backing, they are backed by their own internal mechanisms of scarcity enforcement.