"Dark forest" doesn't work from a game-theoretical standpoint. It assumes that it's cheap to destroy entire planets and solar systems. Yet no-one seems to be colonizing other systems.
The optimal strategy in this case is to expand as quickly as possible, before others can grab or destroy resources.
I think one idea is that there aren't very many habitable planets. The Trisolarians are coming to Earth because it's their only option, not because they want to destroy humanity.
In the book, the trisolarans aren't coming to earth just because is the only habitable planet, it's because it's the closest one too. The don't have FTL and they spend the full resources of their dying civilization to build a fleet that will reach earth in ~400 years.
So yeah, a lot of prime real state but no way to reach it.
400 years is nothing. At this speed, all the Galaxy would have been colonized long ago.
This subplot in the book is at least reasonable. Somehow aliens capable of pico-scale engineering can't make a telescope to observe the nearby solar systems, but OK, whatever. Cultural blindness and all that.
It's the later plot with the "dark forest" that makes no sense. There are ships capable of moving at relativistic velocities and spacetime-altering weapons.
If you're colonizing systems so stealthily that it's not visible, then why bother doing that at all? Even a single solar system has more than enough resources.
In the 24th century, 8 million people flee to distant Safehold after barely escaping from the destruction of civilization by a genocidal alien foe, the Gbaba. Because the enemy tracked signs of advanced technology to hunt down other emergency colonies, the plan is to enforce a pre-industrial society until the danger passes. Administrator Eric Langhorne, a megalomaniac and neo-Luddite, instead has the colonists' memories erased and replaced with the belief they are the first humans, "Adams" and "Eves," the creations of God upon Safehold with "Archangel Langhorne" anointed as prophet and leader of the Church of God Awaiting.
Entropy and thermodynamics exist. It would be impossible to colonize every system we see without the tremendous heat energy expenditure needed to do so being obvious, and obviously not natural. The background temperature of space is so regular and predictable that any deviation is noticeable.
Barring some magical woo-woo technology that someone will probably insist must exist because we foolish humans are simply too stupid to understand anything, this isn't something you can just cheat. There ain't no stealth in space[0].
its a few years since I read Liu Cixin's books and even longer since I read Greg Bear's, but the technology was very different, and it was not as uniformly hostile. The primary technology used to destroy civilisations in Bear's books was self-replicating machines, and there was an advanced civilisation that tries to defend or rescue those attacked, and punish the aggressors - so its a bit less bleak.
