What would two infinite parallel mirrors reflect with nothing between them?

[HardDay'sNight]

If two planes of mirrors (infinite in all directions) were facing each other with nothing in between what would they reflect? How does light come into play in this?
If they were not directly stuck to each other (in which case there would be absolute black, no light I assume) but at a distance apart?

 

[Danger]

Just my thoughts, not an expert answer.
I suspect that they would each reflect a slightly smaller version of the opponent... indefinitely. The end point, I believe, would be when the resolution of the image exceeds the resolution of the mirror.

 

[cesiumfrog]

The only sensible answer I've got is "any photons already there would bounce back and forth between the mirrors". There would be more to say if an observer and light source were included.