Why is the cube visible even though there are no lights? If I use the Cycles rendering engine, leave the default Cube, delete the default lamp, set the diffuse color of the cube to some color, why will the cube without any lamps still render visible? Does the default background emit light? Where can I tweak the amount/color of light it emits?
The background is emitting light (by default). You can see the setting of the world background in the world Tab: