Dark is the absence of light. Dark would only take over when the light source is blocked, turned off, or otherwise stops emitting light. Dark would take over at the speed light as the photons of light continued moving.
Speed of dark?

Isn't 'dark energy' an un-observed correction factor to explain the speed of objects in space? In other words it is only theoritical at this point?

Yes it's still theoretical more hypothesis stage i believe, explanation for what they are observing and for a lack of a better way of explaining it properly just call it that. There is an unseen force operating which that they tell but can't fully understand what it's significants is
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I saw this interesting question--what is the speed of dark-if it even has a speed?