I can tell you what is 'supposedly' behind the white light
It appears that what is behind the light is an alien world.
Before you write it off here are two reasons why I bring it up.
1. In the DVD commentary, the director says there was originally a scene in the film where the people in the cube find food that is not like any seen on Earth. He says this scene was removed from the script because it revealed too much about the cube.
2. The second comes from a Cube movie forum I visited several years back. Apparently a few people from the forum had seen the 'destroyed video' that had the extra scene after the autistic character escapes. The film had him walk emerging from the light on to a beach. Behind him was a high tech door built into a cliff face. The door then (apparently) disappears. Kazaan then looks around more closely at the beach. Apparently it was of incredibly white sand that reflects light as no sand on earth does. The ocean is purple and makes metallic grinding noises. The sky has several moons in it, despite being day. Fade to black as the machine noises grow louder and louder.
Now let me explain the second point here. For all I know, this could be a complete lie. However, the only support I can give to it is that I saw the same thing posted on a different forum many years before I visited that other one. So, it may be true, but it could still be a lie. I cannot support it with any evidence and if you do not believe it I cannot blame you. I am simply telling you what I saw posted in two other places.
You can check the first point on the DVD though.
As for Worth's comments about human constructing the cube through contractors... I have nothing to offer. I don't really buy the alien explanation myself, as it feels too awkward for me to accept. The only thing I can think of, is maybe in an earlier draft of the script Worth's comments were not included, but after filming began it was added in and the original ending scrapped. But that is just guessing on my part. The guess assumes the video described above is true.