A "sector" of the Standard Model---like the QCD sector, or the electroweak sector, or the Higgs sector---is just shorthand for a part of the the Standard Model that consists of particles and forces.
It's possible that dark matter is just like Standard Model matter, in the sense that it only talks to a force that we already know in the Standard Model, like the weak force. But it's also possible that there are other forces. The "dark sector" is short hand for 'whatever stuff is making up the dark matter, and however they interact'.
It's possible that dark matter is just like Standard Model matter, in the sense that it only talks to a force that we already know in the Standard Model, like the weak force. But it's also possible that there are other forces. The "dark sector" is short hand for 'whatever stuff is making up the dark matter, and however they interact'.