All money is used to pay debt, just gold backed (or money backed by anything tangible), is not created without the tangible asset there that it represents in the simplest form of an asset backed currency.
What we have now, or a fiat money, is money that is only created by being borrowed into existence, and so is said to be created 'out of thin air'. And since it is borrowed into existence, repayable only with interest, so the money supply must by necessity constantly grow to pay the interest on the money that has already been created.
It takes a while for most to grasp the concept but that is truly how money and by extension the economy works (or doesn't) today.
It is not that "currency is partly backed by ephemeral financial instruments that nobody really understands." Believe me there are plenty that understand this very well, Bernacke, Volkner, Greenspan, I'm sure are all crystal clear about how money is created.