I wonder if network has a site for everything.
Or are there topics which are not covered by stackexchange?
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There is (or, at least, was) a process for new Stack Exchange sites to be created, using a site called Area 51.
One of the main people in charge of Area 51 was fired earlier this week, so whether things will change moving forward hasn't yet been clarified, but in general, it is safe to say that there is not a SE site for every topic.
Rather, there is a SE site for every topic that a SE site has been created for.