I wonder how a digital asset could be created without a central trust where the real assets are stored.

A very simple mechanism for a digital asset is to set up a trust that  associates for each asset, e.g. shares IBM, a digital Bit Certificate that establishes the link with the IBM shares.

So in essence for each real asset a digital currency is issued which must be backed up by the real asset for 100%. The only advantage is that you can exchange this currency without brokers and store it decentralized.

Once the companies itself start to issue Bit Certificates for their shares the need for the trust disappears.

To organize a derivative market their is always a market coordinator needed between the buyer and holder of derivative contracts.