Is there an existing protocol where a sender does not know who he is sending data to? I was thinking about how the BitTorrent protocol automatically gives every peer a list of IPs of every other user who is currently seeding / leeching off the torrent in question. Is there an existing protocol where, somehow, the destination address is vague enough or unknown to the sender (and perhaps becomes more known as the packet is sent through the routers ?) Or is there research in implementing something like this that could be interesting to read ?
If you want strong protections for the identity/anonymity of the participants, you might be interested in systems for anonymous communication, such as Tor.

If you're just interested in systems where the destination's address isn't necessarily known under normal operating conditions (but it's not a critical security goal to ensure it can never be known), you might be interested in peer-to-peer overlay networks, such as Chord and many others.