This has meaning on two levels.
First, literally, Jack is having that feeling that we all feel in the dark when we can't tell if someone else is there. We almost believe there is someone because we have fear of the unknown. This may mean that while Jack's hunting, he is actually feeling the presence of the pigs who are escaping his line of sight.
Second, figuratively, Jack is feeling the evil of the place. Many different literature texts refer to evil as something tangible. In fact, "The Most Dangerous Game" is also set in a thick jungle on an island. Most Puritan literature references the evil that lives in the forest. These unchartered places were considered the residence of evil. Eventually in Lord of the Flies we see this belief come to fruition because the kids are influenced by the evil and begin having no concern for human life. Likewise, they give evil a vessel in the pig's head that they place on a stick as a symbol. The darkness and evil could be that which is the "something behind you" that Jack references.