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The dark frame is a representation of thermal noise in the ccd or cmos chip when the lens is capped, so to speak. As no light is striking the chip, what you are seeing is the thermal noise generated on the chip, as the chip pixels or charge wells register charges as if a photon has been captured at that point (as well as other artifacts such as cosmic ray strikes).
Thermal noise is halved with every 5 to 6 deg C drop in temperature, so you should try to capture dark frames at the same temperature as your light frame to avoid a mismatch. My SBIG ST7E ccd camera sets and regulates the temperature to within ± 0.1 deg C making it easier to capture darks at the same temperature as the light frame.
I once mistakenly subtracted a dark frame from a light frame at different temperatures, ending up with a final image spattered with black holes where “hot” pixels were subtracted which shouldn’t have been.
Cheers
Dennis
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