Photo resistor is light sensitive.
There is an item named "Dark resistance" as shown on specification,
For example, for MF5510, the dark resistance is 0.5M Ohms.
One thing to remind, what will happen to the darkness resistance when The cell has stayed in total darkness(after closing 10lux lighting) over 10seconds?Actually, the dark resistance will increase and will much bigger than 0.5M Ohm.
And the end user normally want the dark resistance goes high when in darkness,
And the minimum is what they concerned, here is,for MF5510, Not less than 0.5M Ohms at 10th seconds after closing 10 lux.