The Dark Forest makes so many (explicit and implicit) assumptions it starts to feel extremely dodgy. There are also falacious arguments embedded within it which fail to imagine numerous possible scenarios simply by following "tried and true" paths copy-pasted from sci-fi novels.

TLDR: The forest analogy fails because civilizations are not singular entities (like animals are) but rather complex structures which can merge and evolve. The supposed technological and societal barriers can very well not materialize in which case the theory falls apart.

Details below...

> Suppose a vast number of civilizations distributed throughout the universe, on the order of the number of observable stars. Lots and lots of them.

"On the order of observable stars" is extremely wide. Perhaps there is one life-bearing planet per galaxy. Perhaps there is one per 100 stars. Those are vastly different outcomes.

> Survival is the primary need of civilizations.

On individual level humans have no hope to "survive" (i.e. live immortal lives). Yet despite that fact we don't commit mass suicide. The "primary need" of human civilization to survive is merely emergent behaviour due to other factors. Perhaps advanced civilizations would be happy to achieve some grand undertaking and then perish.

Now for the second "axiom":

> Civilizations continuously grow and expand ...

Expansionist civilizations with unlimited space certainly can do that. But does it really bring any benefits? The more complex the societal structure, the harder it is to change. Perhaps there is a critical mass above which it becomes detrimental to multiply beyond and all "sane" civilizations will stay below it.

> ... , but the total matter in the universe remains constant.

It may be finite, but perhaps most of it will be "stranded" anyway - so far away that access to it becomes non-feasible anyway.

> ... In other words, the finite nature of resources will ultimately pit one civilization against another as they all struggle to sustain their growth.

This part I believe is the biggest logical fallacy of all. What is civilization if not a set of societal structures? And we can easily imagine merging of two such structures - they are already composed of other structures; what is just another level of complexity? If we merge two "competing" civilizations suddenly we have a single larger civilization... supposedly without conflict? The amount of resources is the same as before but not we only have a single contender: the merged civilization.

This paradox stems from the failure to account for the internal race for resources inherent to all civilizations. The civilizations are not unitary entities. Treating them as such and assuming blind dedication to it from its members is short sighted and lacking imagination.

> If open communication exists, then conflict could be averted and tensions diffused. But given the time-lag involved with any attempt at interstellar communications, civilizations that are light-years apart are unlikely to resolve their fears quickly.

Sufficiently advanced civs can simply send AI envoys or do mind upload of their representatives with the speed of light. This makes the learning of language, communication etc. very quick. Perhaps FTL communication can exist too. Etc. etc. Endless ways to break another assumption.

> By the time they arrived, the attacker’s technology would not have matured one bit, while the defender would have decades or centuries to progress.

It may be possible that advanced technology does not progress as fast as it does now. See our experiments with fusion and particle accelerators. What used to be easy now takes years and billions in funding. Perhaps going at some point getting any kind of advancement will be nearly impossible and all technology will become "old". In this scenario the centuries of technological advancement are moot. OTOH potential gains from trade from foreign civs is very real possibility.