Everyone has domain knowledge. The guy in the self-storage warehouse, the History teacher, the researcher. Anyone can gather domain knowledge. It’s based on experience. The value comes in adding value to that domain knowledge.
How can we do that?
The first way is to share the deep, specialised knowledge that you’ve got. Give it away. Know about obscure 1980s Japanese comics? Share it. Help people. Found a cheap way to fix that long-discontinued engine that a couple of people you know have been having problems with? Likewise.
The second way is equally important. Seek out new domains. Find synergies between the two domains. Find metaphors and procedures from one that fit the other. Insights come through immersion and reflection.
Can you think of a third? :-p