I place a sun light in the scene, and the window stays pitch black. I notice there is a portal in the window. A portal to a black hole :P? Some light transmits through though, but none of the world light.
I'm using Blender 2.79. I think i figured it out though. The world background strength was set to 0 by default, resulting in a pretty black scene no matter how bright my sunlight was. I have done a couple of tutorials on rendering in the past and can't recall touching the strength. Just the colour of the ambient light/background.