Fan Theory: The Light is Time.
The Egyptians partially blocked and channeled the light (i.e., time) and began to manipulate it. In fact it appears that the Egyptians installed some sort of valve device that could alter the flow of time, depending on how the cork was placed in the device time could be slowed, sped up, or even regulated to flow normally (as it was in the end of the End). The instant that the Egyptians manipulated time there was two serious side-effects. First, the past, was no longer just a memory. The past became an actual entity because now the past exists in more than just the mind, now the past actually exists in the real world because by manipulating time you are not allowing the past to be “let go” (by creating the ability to travel back to it). Ultimately the entire message of the show LOST was our struggle to let go of the past and not be consumed by it, yet LOST took this concept a step further and designed the show around our character’s battling their pasts in a very real way. In LOST the flash backs of our characters not only told the back story but also helped to define the monster which was a combination of all of their pasts and others that had come before them. Thus, the birth of the smoke monster was a side effect of the very first manipulation of time.
Therefore, the ancient Egyptians were the first to manipulate time, but in doing so they created the smoke monster which is really nothing more than a physical version of the past.
The light = time (the future, hope, etc.)
The smoke = the past (the exhaust of our lives)
This is a more elaborate version of the "second chance" theories, wherein the main characters were brought to the island to have a "second chance" to redeem their inner fears, demons or mistakes. However, this theory postulates that the way one can re-live the past is to actually go back in the past.
The ability to harness the "power" of time would drive men like Widmore to do anything to get their hands on the island. By being able to control time, a person could be immortal, rich and powerful. One could move people around the globe like chess pieces to change past events to re-write the future.
The explanation that the ancient Egyptians found the source of time, as a tangible thing, tries to tie in the Egyptian materials shown throughout the series. The Egyptians were highly advanced in engineering, mathematics and astrology. Further, ancient people did not view time as a linear constant that modern mankind thinks of time today. The ancients thought time was cyclical, like the seasons.
The idea that the smoke monster was the creation of the first attempt to control time (by embodying the past in a cloud of smoke) appeals to the clues that how did the smoke monster manipulate itself by creating past memories for Jack (his father) or Kate (her horse)? The smoke monster was more than just the concentrated events of the past, since it could also manipulate matter to create human beings and interact with others in present.
If LOST was merely about letting go one's regrets, that seems to be an underwhelming lesson for the show. Because not all characters seem to redeem themselves or change their ways. It also does not fully explain why some characters died on the island after confronting their past regrets instead of living forward in the present (like Boone, who found out that Shannon no longer needed him and vice versa, was killed by falling off a cliff).
The light and dark themes could be time (present) and the past. It could also represent in Egyptian culture life (light) and death (darkness) for the Sun god, Ra, had a nightly journey through the underworld only to rise at the dawn of each new day.
Even those who may have understood the light being time, were not able to control it. Turning the FDW teleported a person backward in time to the North African desert. Eloise Hawking had trouble finding the island since it may have been moving in both time and space. If she could not control the island, who could?
