Given that this site is “Geoff Does Stuff” then it would seem good to define what is meant by “stuff”. It is often the case with English words that they have more than one meaning and stuff is no exception, so if you want to know more of these then take a look at Chambers Dictionary but from my point of view it means this:
task, without a well-defined owner that needs doing, usually by someone who “does stuff”
Although I would add that stuff can be used to mean a single task or multiple tasks. Generally, this kind of task is expected and known about, so when you start your day, you know what stuff you need to get done, but then......
Well the dark stuff is all the unplanned and unexpected stuff that you do that gets in the way of the stuff you were supposed, or planned to do.
The Dark Stuff is so called because it is stuff that needs to be done or else things go wrong, but it tends to be stuff that other people are oblivious to and that is essentially invisible to them. However, they know (or learn) that if there isn’t somebody there dealing with the dark stuff then things go wrong. It’s an analogy to dark matter. We know there’s something there, but it can’t be seen.
Dark stuff is often not understood but it is well known to regular “stuff doers”. In fact, we often don’t understand ourselves until we see the thing that needs sorting out, but others don’t see these things at all.
I will close with what I see as a classic quote from an Information Technology Project Manager:
I don’t know what Geoff does, but I know you need him on your project