In the particular field I'm in (radio astronomy), I usually have two choices when hiring. 1) Hire someone with radio astronomy knowledge and teach them to be a good software engineer, or 2) Hire a good software engineer and teach them a bit about radio astronomy. My point being, you don't have to know everything. From my point of view as the person doing the hiring, you simply have to demonstrate a capacity to learn coupled with a well-groundedness in the basic concepts required for the job.