I'm building an interstellar empire that uses commodity money as its preferred currency. Despite the network of wormholes connecting various star systems, light lag between planets and stations within systems makes centralized digital transactions inconvenient. In addition, the empire's computer technology is limited to the equivalent of Earth's in the 1970s, taking cryptocurrencies off the table. The empire also wants to minimize economic inflation, due to its strict mantra of peace and stability (and consequently stagnation). All of this has lead it to choose commodity money and commodity-backed money as its main means of commercial exchange.
However, there remains one problem——elemental gold is dirt cheap, as is silver and copper. Asteroid mining has been a common practice for centuries, making most such metals that are traditionally valuable on Earth nearly worthless. Faced with this issue, what would the empire's coins be made of? The materials would have to be stable, with a half-life of thousands of years at the very least, and of course be safe enough for constant handling by humans. Are there any currently known metals that fit these properties and are rare enough in the universe to be a gold standard? If no such metal exists, are there any viable nonmetals that fit the role, or even any forms of exotic matter?