There are many threads, it's to do with the lighting model and the fact that only one light source can be used at a time to light planets. It's been here for ages but it may be changed in 2.4 when they add support for multiple light sources. When the sun lights the side of planet the ambient light level is set by the lit side, this unfortunately results in a high ambient light level on the dark side. The best way to see this work is to visit a small moon during an eclipse, you will suddenly find the entire moon pitch black because the daylight side is now unlit and the ambient light level drops to zero.
If they start to use multiple light sources the light level of the night side of the planets and moons can be calculated separately from the daylight side taking into account the available light sources. As mentioned there is also the light from the skybox, but this is only used as the primary light source when you are on bodies far from the main star, it gets even more complicated with multiple stars because only one of these is used as the light source.
The multiple light source update has been rumoured to be very graphically intensive, so only high end PC's may be able to take advantage of it.
