"But who created the universe" -> Must be a "God"
"But who 'backs' Bitcoin" -> Must be a person, business, physical object (a yellow metal) or gov't entity.
If we had to place a persona behind Bitcoin, we'd probably say that it's "backed" by the people, or the users. A more interesting question would then be "why is it backed by the users?" Reason and Logic, which lead these users to believe that Bitcoin has intrinsic value as a technology.
Both Gold and Bitcoin are backed by the same entity, the free will of the people that choose to use them and the value the people place on them.
Gold is decentralized in the same manner as Bitcoin. If I have a piece of Gold in my possession, that piece of gold is out of the hands of any central system or government and fully in control by myself. Under a Gold currency system, gold is distributed and controlled by the hundreds of millions of people who each possess a piece of gold, not a central government. And it is these hundreds of millions of people who determine the real value of each unit of Gold. Similarly if I own the private keys to some Bitcoins, those Bitcoins are fully controlled by myself and out of the hands of any other entity.
Bitcoin shares more attributes with Gold than I think most people understand, but which will enable Bitcoin to reach some amazing heights. (Provided the network holds and there are no successful attacks)
BUT, various people can each own the same Bitcoin at the same time (just share your private key) and a number of people can own Bitcoin but only jointly (multi-sig). These things are not really possible with something physical such as gold.
You can have the same ownership / control with Gold as well. For example you could store it an account where multiple people are established as owners and who all are required to approve actions, or store it in a safe known only to a number of people where each person's individual key is required to open. This provides the same multi-owner control over Gold.
I agree it is easier to setup this situation with Bitcoin than Gold, but my point that both are decentralized systems because ownership is decentralized holds. It is this decentralized nature of ownership which made Gold such a strong form of money over history, and which will make bitcoin a strong form of money as well.