This is a heavily speculated thread so feel free to disagree with some of my points, use them to expand upon your own or elaborate on them. I just please ask that if you should choose to disagree, to please back your point up, I want to learn as much as you do :) so introduction aside this is a hypothetical question in which I try to answer it myself with reasoning, warning, long read: Question: If the Vex can time travel, why don't they just travel to a point in time before we used the Traveler's power to wipe us out then while we had next to no defenses to kill them? That question established let's delve into it deeper. I want to take a look into the Axis Minds: [quote]How like you to ask me for the bad news, even in this moment of triumph. I've finished going over that Ghost's report. It is my hypothesis - a hypothesis at best - that the Vex saw the abominable presence at the heart of the Garden as a divine power. I can hear your protest already: how can machines have a god? The answer is simple. The Vex, for all their voracious intelligence, could not understand or decipher what they found. They searched through all available reactions, and they settled on the course with the greatest payoff...to worship this power, and to remake themselves in its image. I believe the three Axis Minds found in proximity to the abomination were Vex machines built to serve as vessels for this power: a way to extend its reach across space and time, binding it to the Vex, and the Vex to it. If they had succeeded, I cannot begin to guess what horrors they would have unleashed. Attend carefully. There is cause for hope. When endangered, the abomination activated these vessels and defended itself. This tells us that it was threatened. Whatever it was, Guardians could harm it. And it activated only a single vessel at a time. Its strength was limited. Whatever it intended, it was not ready yet. We must assume the abomination was part of something greater. And we cannot flinch from the terrible, obvious comparison: just as the Traveler acts through us, this power was able to act through its own servants. Let us be wary. There may be other abominations, and other vessels.[/quote] From what this Grimoire card tells us is that the Vex found something they couldn't understand at the Heart of the Black garden, and for a race of hyper intelligent automatons had simply no other choice but to worship it. I theorize that the black blob at the heart of the BG was a part of the Darkness, not just simply a part, but the Darkness' answer to the Traveler's Ghosts. This blob was able to animate a statue which each gave off shadowy auras when animated. The grimoire also speculates that more than one may exist, more than one pieces of this Darkness, just as more than one Ghost exists. Killing this fragment would "injure" the bigger darkness as much as a Ghost dying would "injure" the Traveler since that is a piece of its light being consumed. The point of the above tangent was to elaborate and emphasize that the Vex have very good knowledge about their adversaries. We also know that the Vex are a giant Hive Mind, Ghost says this in one of the Venus story missions. The Black Garden is also the speculated "home" of the Vex: [quote]The Garden grows in both directions. It grows into tomorrow and yesterday. The red flowers bloom forever. There are gardeners now. They came into the garden in vessels of bronze and they move through the groves in rivers of thought.[/quote] The gardeners are clearly the Vex as they are vessels of "bronze". Also what Pujari's Ghost says is interesting: [quote]The Ghost said to me: You are a dead thing made by a dead power in the shape of the dead. All you will ever do is kill. You do not belong here. This is a place of life.[/quote] The garden is a place of life, as in does it create life? It is speculated the vex originated here and we know that a piece of this Darkness resides at the heart. Ghost also tells us that the Garden is in a place of no known space or time which means it could've been in an entirely different dimension, which is not silly is one looked into the Vex weaponry of say the slap rifle: [quote]From a tactical perspective, the Slap Rifle is a Vex directed-energy weapon that fills their analog of the light infantry role. From an engineering perspective it's something much more interesting: a terminal. The Slap Rifle receives a bolt of Solar energy from somewhere (or somewhen) else and it points it at a target.[/quote] Which could potentially mean that the Darkness inadvertently has the ability to cross spaces in the form of dimensions if the aforementioned postulate is true. So the Vex worship the Darkness and kill its enemies on top of their own goals for expanding, and let's for the sake of discussion, and this can be a big hole in this topic but I'm willing to take the risk, say the Vex are somehow a spawn of the Darkness as they originate from the Black Garden and a piece of the darkness lies at its heart. This established, let's finally go back to the original question, why didn't the Darkness make the vex go back in time to kill humanity while we were incredibly vulnerable? Hypothesis: The Darkness' goal isn't to wipe out humanity, it's to wipe out the Light. The prologue of the game tells us the Traveler had "an ancient enemy", the Darkness, that had chased it for eons. This would mean it chased it across multiple star systems and potentially across thousands of races. Keep in mind, technically, we went to the Traveler, it didn't come to Earth, it went to Mars, where we then went to it. not to say it probably wouldn't have went to Earth, but we do not know how it operates or if it is truly sentient, so it is basically a deity to us....as the darkness is a deity to the other alien races. if the Darkness' true intent was to kill all life, why then do the Fallen and cabal exist? We know they have history outside of the Darkness as you can "follow a trail of dead planets back to their empire." (The Cabal). The Darkness grants the enemies of humanity power as the Traveler grants the allies of humanity power. So where am I going with this? The Darkness represents entropy, the natural state of things in the Universe, the Traveler represents order, and it is entropy's job to disrupt that order. The Traveler and all it's "Light" is what the Darkness was after, and by using that Light, we "marked" ourselves for death akin to how White Blood cells kill pathogens that have been marked for death by the immune system. We attracted the Darkness to the Traveler's Order. In my opinion, it didn't see us as humans and killed us for that, it saw us as a piece of the Traveler now and killed us for that. So why didn't the Vex travel back to wipe us out? Assuming they follow the Darkness' orders, they didn't kill us in the past because they didn't need to, the Darkness didn't "need" to. We had no light, no Traveler and didn't perceive us as a threat until we mingled with the Traveler. The fallen and Cabal didn't mingle with the Traveler and they are still very much alive and have come into contact with the Darkness. The hole here though is that if the Darkness is truly sentient, it would somehow make the vex kill us before we could use the Traveler. But, in my opinion, it is just simply a higher entity that only does things on a reactionary level, that reaction being the activities of the Traveler. UPDATE: I forgot to mention how Ghost says the Venus ruins predate humanity by billions of years, meaning the Vex had copious amounts of time to even wipe us out then