The origin of the Light (and the Darkness) in LOST by Haferflocke

Short answer: It happened during the incident.

Explanation:

The key is the song "catch a falling star" that you hear again and again since the first season of LOST. The first two lines are:

Catch a falling star and put it in your pocket
Never let it fade away

I think, this reflects Juliet (who got literally branded with a eight pointed star, which symbolises resurrection, rebirth, and renewal, on her lower back in season 3) falling into the pocket of energy. (more about juliets mark: http://lostpedia.wikia.com/wiki/Juliet%27s_mark)

After she and Sawyer insured each other of their eternal love, she finds herself at the rock bottom. In her darkest moment, she hits, whether in desperation or inner strife or the desire that she and Sawyer had never met, so that she could not lose him, the bomb and it detonates.

Whether it is a quantum effect of the explosion, eternal love, her consciousness in a very special state or a combination of them all, this is the point at which the eternal light manifested beyond the rules of time. But also the darkness of Juliets shadows.

Pay attention only to the noise in the scene when Juliet becomes aware in the abyss, you will find all the contradictorily sounds that the smoke monster causes. Even the rattling of chains, that wrapped Juliet and dragged her into the abyss.

So, the fade to white in the last scene of the finale of season five is not just a cliffhanger, it is the birth of the eternal light, unbound in time.


Sorry for my bad english. ;(