Your dentist is right in that post-extraction antibiotics are not justified for a simple extraction under local anaesthesia. The risk of post-operative infection is very low. The white substance is quite normal, and is a type of granulation tissue forming within the socket that will eventually re-organise to form new gum and bone. One of the main indicators of uneventful healing is lack of pain. 'Dry sockets' are not that common either, and typically come on a few days after extraction. They are characterised by increasing pain, and bad taste and smell. Even if this occurs, antibiotics are still not warranted, and your dentist can irrigate the socket and place a sedative dressing if needed. If you rinse CAREFULLY so as not to disturb the socket, and avoid smoking, the chance of a dry socket is very small. Hope this helps!