Very good question. It seems obvious but actually, it is very important to understand this. Even real money that we use every day is not totally backed up. Countries can have their own money if they have a certain quantity of gold. Money is backed up by gold. Unfortunately, even the dollar is not strong enough when it comes to real money. Remember the subprime crisis? It has everything to do with virtual money and real money. Nowadays, recognition of bitcoin is hard. With time, more and more companies will use virtual money and maybe then we will not need any real money to back it up. It can work on it own.
Well, money was backed by gold until Nixon removed the gold standard in 1971, which meant that it no longer backed the dollar and they could be directly converted. That then enabled them to go into full force manipulation as they started printing as much as they wanted. Which, as you said, is a reason for the subprime crisis (along with a lack of accountability of banking executives as they got away with selling rotten meat for AAA prices).
As said above, investors back bitcoin. Also, blockchain backs bitcoin. These are the only two things that can stand confidently behind it and hold its value. But it doesn't have any kind of instant convertibility like the gold standard had on the USD.
just like you and me, we are the ones deciding it's value, not the government not the dollar not the gold, but real people who are in this business. 
