nothing needs to back it since PMs are finite..you can’t just generate it from thin air.
I propose a currency that’s backed by empty beer bottles. You can’t generate those from thin air, either. We’ll set the exchange rate equal to $100 for one empty beer bottle.
(sarcasm off)
The fact that PMs are finite, does not instantly make them valuable. There’s not much intrinsic value in PMs. Also, unless you live in Zimbabwe, being able to generate money out of thin air is not a problem. In fact, 99 out of 100 economists will tell you that it’s a good thing.