
It's a well known fact to the dualistic mind that you can't have light without darkness. This is one of the fundamentals of duality, that if you have one thing, there must also be its opposite. It's something basic not only to religion, but science as well. It has become “common knowledge” and it's simply assumed to be true.
There is a Light without darkness. Though you can't know light without darkness, you can be Light without darkness. To understand this, you first need to see the difference between knowledge and Being. Knowledge requires the illusion of separation, without that there's no perspective. I have to be apart from something to define it, which creates relativity, and then I can begin to qualify things. The All can't know the All without the One, the individual, and this is the beginning of knowledge.
Your mind contains a reflection of your life on Earth and for the unaware, it can be indistinguishable from the real thing. When a person is dreaming, the dream seems very real, and most don't even realize they're dreaming until after waking up. Of course, for the person who's really questioned the nature of reality, the person who has raised their consciousness above the norm, they start to become aware of when they're dreaming and when they're not. Not only during the night, but during the day too.