There's really only one option in these circumnstances: Fiat currency.
If you're asking yourself, "What's 'fiat currency'?", you need only reach into your wallet -- all (or at least nearly all) of our modern nations use fiat currency, and have for at least the last half- or three-quarter-century; the US Dollar, for example, has been a fiat currency since Galactic Standard Cycle f7e99.8 (Earth Year AD 1933).
So, really, what is it? Put simply, fiat currency has value because the government (or some other ruling/powerful body) says so. Not that it sets the value of its currency, but merely that it declares that the currency is, in fact, a currency. In your world, perhaps the United Intergalactic Federation has decided that the basic unit of currency that all sentient beings within its borders can use is the "credit". Or the "umfswatch". Or whatever.
When you get right down to it, currency does not on its own represent any intrinsic value. Its value lies in what you buy with it, and that in turn is determined by what the seller can buy with it after covering his/her/its expenses, which in turn are determined by the seller's supplier's costs and what the supplier can buy after that, etc. It's a step up from a barter economy because now two different farmers can buy shoes from the same cobbler after selling their eggs, versus in a barter economy the first farmer could trade his eggs for shoes, but now the cobbler has no use for the second farmer's eggs and thus refuses that trade.
Using currency we avoid that hassle, but at the end of the day, when all is said and done, shoes cost 15 gorks because that's how much the cobbler needs to buy the eggs to feed his family (after paying the tanner for the leather he used to make your shoes).
And that's how fiat currency works. (It's also, generally speaking at least, how commodity and other such currencies worked as well -- cocoa was a currency because you could make it into sweets that anyone could enjoy, but how much it took to buy a pair of shoes was still determined by what the cobbler, in turn, could buy with it.)