Let's be real, fiat currencies are essentially backed by Uranium 235 and defense industry engineering capabilities. (both as it relates to turning the U235 into large amounts of energy in a small amount of time and producing other things like aircraft carriers)
funnily enough you CAN actually redeem the banknotes for these things but most people don't typically enjoy the experience
agree. the USD dollar is backed by the threat of violence which is how I understand what it means to be an empire; in contrast with a 'civilization' (though it seems as though empire is the early stage of civilization, it boots things up)