I believe - and I'm new to this too- that the dark shot accounts for any bad pixels in the camera's ccd/cmos ( defects)and subtracts them from the shots you have taken. Temperature has a lot to do with it as well. If you have been taking 60 sec shots then you should be taking - subtracting- the erroneous pixles of similar shots at the same exposure to cancel out those naughty pixels.
The WHY:
The Dark contains that info to eradicate - subtract ( to some degree) those bad pix at the same time/exposure etc you have made your shots.
At a different time/ exposure/ situation you're camera might not have the same bad pixl's. I have tested this by taking a dark at 60 secs and 600 secs. Totally different.600 gave me a few little red crosses.
I'm sure somebody else will give a more technical explanation, but this is what i have learned .
Bartman
