@Adam_Cadmon1 I think it's a mixture of memory and imagination, just like dreams.

I suspect that we "see" them by our brains simulating the signals of sight that normally come from our eyes, in effect rendering the images for us. The fact that we can see these images more vividly when we close our eyes is what leads me to that conclusion.

When you read about how much of what we "see" is really just filled-in-patterns from our brain supplementing blurry shapes from our actual light receptors, it makes sense that our brains can extend that power to things that are currently unseen.