The biggest thing backing precious metals is a more world wide belief that are a store of value. Meaning that the dollar is linked to the US economy and so will rise and fall with it. Gold is linked to the world economy, and will rise or fall with it.

Also Gold has some intrinsic demand from industrial and personal uses. Be it jewlery, cell phones, certain electronic devices or whatever. Silver was used for a long time in film development and is still used in other industrial processes. What is a dollar bill used for, other than to buy something? Wall art at cheap resturants?