In ubiquitous terms, darkness symbolizes a lack of knowledge and understanding, an absence of civilization. When colonialists, for example, first ventured into Africa, they named it 'The Dark Continent,' since they believed its inhabitants were uncivilized savages. It is, in this sense, the exact contrast of light which would represent understanding and civilization. Darkness is also associated with evil. The forces of darkness are deemed to be evil and malevolent in opposition to that which is good and wholesome. The epitome of such darkness would be Satan, the lord of evil.
In Lord Of The Flies, darkness symbolizes all of these. Firstly, the boys' descent into savagery means they lost their civilized behavior and devolved, thus embracing the darkness. It is as if they forgot what they had learned and embraced the darkness. In doing this, they became evil, thus also welcoming their true malevolence and giving vent to their innate ruthlessness. They became uncaring toward others, seeking only to satisfy their lust for blood.
Both Simon and Piggy recognized this innate evil. When the boys were talking about the beast, their greatest fear, both Simon and Piggy commented in some way or another about the possibility that the fear they had, epitomized by the beast, was something within them. This is proven when the boys give vent to their lust for blood by reenacting the hunt, celebrating the slaughter of the pigs they hunted; when Simon is killed on the beach; Roger's intentional murder of Piggy, in which Roger deliberately dislodged a large rock, which hits Piggy over the edge and destroys the conch; and the eventual hunt for Ralph.
In addition, the boys' uncertainty and fear are epitomized by their fear for the beast, which is a creature of darkness, something that one of the littluns believes comes from the sea, and something that others believe they heard during the night. Jack mocks this belief, since he thinks the boys' fears are the result of their overactive imaginations and nightmares. It does, however, seem as if this fear of the unknown is more real than Jack wishes to acknowledge, and he is eventually forced to seek the source of their fear, accompanied by others, when they go to find the 'beast.'
Simon, during one of his hallucinations, encounters the 'beast within' and discovers that there is no real escape from the darkness within oneself. He learns that whatever one does will only lead to one's destruction. In the end, he becomes probably the most profound and tragic victim of the darkness, which has overwhelmed the boys.