Space and alien imagery
The imagery that most defines The Dark Forest is the imagery of space and interstellar alien societies. This science-fiction element adds a level of hypothetical consideration to the novel. At the end of the book, the reader gets the sense that this could really happen. It is technically within the scope of possibilities, but it feels so unfathomable. The imagery of space is constant, and the development of a space colony and a desolated, unlivable planet fulfill the genre expectations of the novel.
Technology and progress
The future is technologically sophisticated, we know from experience. This happens because as time passes, humans are able to continue building on the insight of prior people who existed before and whose contributions allow a further starting point to the people who follow them. Through a hypothetical device called a "sophon," the aliens are halting human technology and threatening their future. The drama of the novel comes from the idea that technology only progresses so fast, and the Trisolarians seem to be so far into the technological future that it might as well be witchcraft.
Government and power
The government is a serious operation in this novel. The UN is not just a committee for approving the actions of nations; in this novel the UN has actual legal authority. That is to say that the imagery of the novel involves a totalitarian globalized government, and the effect of that dynamic in the novel is simply that it feels more like "the earth versus the aliens," because the novel's portrait of power is more succinct and global. For the opposite approach, a story like the film Arrival could be considered.
Infinity and paradox
Luo Ji solves the earth's most pressing problem without any sophisticated methodology. He just had an idea that was so true that he was able to effectively refute the alien advance. He often contemplates paradoxes and mathematical riddles. The one most pertinent to the alien epidemic is obviously the Fermi paradox. His experience solving that riddle constitutes a major imagery in the novel (The riddle is: Why don't we hear from aliens more often if the infinite nature of the universe implies their existence?). The answer is that it is dangerous to be discovered, as the earthlings have found out. Luo Ji threatens to broadcast the location of the Trisolarian society into the vast emptiness of space, and the Trisolarians agree to a treaty. They don't want universal attention.
