First of all, I want to ask you if Gold is backed by anything, or why do many people think that it is one of the best investment?
Most people say gold has historic significance to it, it's also used in industry quite a bit and is used a jewellry as well as other stuff.
The main thing increasing the price of gold is the miners profiting off it though by producing a scarcity, most industries can use platiumum better than gold too so there's less of a reason for its uses.
On the what is bitcoin backed by, what is currency backed by? There has been a lot of studies on the economy and many have found that the largest investments are in derivative assets (often people buying portions of assets or buying assets against other people selling them - while neither hold the asset - which does seem quite odd).
I've generally considered an investment in Bitcoin to be an investment in its: name, developer community and infrastructure at least.