I'm joking, that's terrible.

I'll give a real answer when I get home.

POST EDIT: I will say your post is interesting, but I think the reason for the whole light ending seems to be symbolic, in my opinion. You're fighting the heart of the Black Garden--a place so dark, it can't even be found. Keeping it locked 'out of time' so to speak can prevent the eroding hands of time from forgetting it or worse, leading anyone to it.

The Vex "Locked the Black Garden out of time". The Gatelords purpose is to manage the Vex teleportation network and guard the realms the Vex "lock out" of time.

But what does that really mean? When you free the Black Garden from the grip of the Vex and the Heart of Darkness all that seems to happen is that the lights turn on and Ghost can communicate again with the tower. It doesn't seem very special.

Likewise Golden Age humanity built a college practically on top of the entrance to the Vault of Glass and the Endless Steps, two major Vex installations. Why didn't they ever enter either? Surely the Vault Door is easy enough to break open given sufficient fire power. If the normal Vex computronium can be shattered by geologic events then some heavy explosives should do in a pinch. Is the Vault also locked out of time?

I think they did. Remember, in the mission "The Archive" one of the things stored in the computer systems is "...mappings of a Vex Underworld, a place called The Vault of Glass..." So either they stood on the sync plates or they got through another way.

If you imagine a 2D graph, with the x axis as position, and the y axis as time, a 45 degree cone contains all the possible locations an object can have and not exceed the speed of light.

However, as you enter regions of increased gravity, the cone starts to tilt due to the distortion in the shape of space. With enough distortions it is possible to have a cone tilted such that an object can move down the y axis, and thus backwards in time.

You could also manipulate things such that a point in space could not be reached, because every attempt would land you back where you started. imagine instead of a series of cones looping back to move in time, they form an S curve. Thus no matter how fast an object moved, it could never reach the point concealed in the S, instead always under or over shooting it.

Thus without the Gatelord's codes, you could never reach the Black Garden because you could not physically move to it. It is the ultimate security. The Vex can then access the locked region via a wormhole/portal without worrying about someone else happening along.

The only issue is from the fluff we know the Golden Age Humans knew about the Black Garden, and that Rasputin fought the Darkness there. It is possible then, the Darkness attempted to use the Garden as a fortress. A place to launch attacks from and then retire behind a nearly insurmountable defense. We know the Darkness can teleport, also from Rasputin's cards. it is possible he found a way to replicate that feat and bring his own doomsday weapons to bear. It also makes sense then why the Darkness would return there after the final battle and not some conventional location like the Traveler and the city.

Bonus question:

In Destiny, is it a many worlds interpretation or a singular universe? The vault likes to use many phrases like "times ocean" and tapestry, that indicate that there are multiple universes like the Sliders TV show. The Future War Cult also has a machine that allows people to view multiple different futures.

Yet the Exo Stranger cards seem to indicate that there is one time line that she keeps looping through. One set of events over and over and over again. How can the Vex Oracles and Medusa "edit" you out of existence if they just keep making an alternate timeline?

Even if they do make alternate timelines, you care a lot about the one you are in and not so much about the others. If you get eliminated from you own timeline it's not much consolation that other yous might still exist. :p

The Vex "Locked the Black Garden out of time". The Gatelords purpose is to manage the Vex teleportation network and guard the realms the Vex "lock out" of time.

But what does that really mean? When you free the Black Garden from the grip of the Vex and the Heart of Darkness all that seems to happen is that the lights turn on and Ghost can communicate again with the tower. It doesn't seem very special.

Likewise Golden Age humanity built a college practically on top of the entrance to the Vault of Glass and the Endless Steps, two major Vex installations. Why didn't they ever enter either? Surely the Vault Door is easy enough to break open given sufficient fire power. If the normal Vex computronium can be shattered by geologic events then some heavy explosives should do in a pinch. Is the Vault also locked out of time?

If any place is locked out of time, I would think the Vault of Glass is. Also who's to say just getting through the door lets you get in? If it is locked away in some special way, I would think you'd need to create a connection of some kind.

If you imagine a 2D graph, with the x axis as position, and the y axis as time, a 45 degree cone contains all the possible locations an object can have and not exceed the speed of light.

However, as you enter regions of increased gravity, the cone starts to tilt due to the distortion in the shape of space. With enough distortions it is possible to have a cone tilted such that an object can move down the y axis, and thus backwards in time.

You could also manipulate things such that a point in space could not be reached, because every attempt would land you back where you started. imagine instead of a series of cones looping back to move in time, they form an S curve. Thus no matter how fast an object moved, it could never reach the point concealed in the S, instead always under or over shooting it.

Thus without the Gatelord's codes, you could never reach the Black Garden because you could not physically move to it. It is the ultimate security. The Vex can then access the locked region via a wormhole/portal without worrying about someone else happening along.

This is interesting, and it felt like a mis-application of theory, but there's FTL travel and a bunch of other incredible things in Destiny's universe, so maybe that is possible.
Of course, it could all be a matter of taking things wrong. When the Ghost mentions it he says "that can't be right", which could just be disbelief at the possibility, but maybe the translation or interpretation is questionable, or maybe it's not what it sounds like, or maybe it's just a theory that got put down as fact.

In Destiny, is it a many worlds interpretation or a singular universe? The vault likes to use many phrases like "times ocean" and tapestry, that indicate that there are multiple universes like the Sliders TV show. The Future War Cult also has a machine that allows people to view multiple different futures.

Yet the Exo Stranger cards seem to indicate that there is one time line that she keeps looping through. One set of events over and over and over again. How can the Vex Oracles and Medusa "edit" you out of existence if they just keep making an alternate timeline?

I didn't take the Stranger's card(s?) as referring to a time loop, just her doing the same thing over and over, looking for something, or (maybe more likely) reliving a memory.

My guess is, multiple timelines, but not fullblown Many Worlds.

Possibly a deal where there's an "Alpha" timeline, and the others are just other bits, cropping out of the wake of the Aether, containing small parts and eventually fading out, or contorting into knots that shouldn't exist, or such.

Maybe they are chaotic or orderly, normally occurring or only made through the effects of the Darkness and/or Light and those that use it/them, and maybe sometimes they usurp the "Alpha", or can be brought in to replace it, or controlled, bent to one's will, and used as a place to store something outside of conventional space and time.

Maybe there is no one main timeline, but there are either multiple ones, or the main timeline is mutable. Destiny clearly doesn't have a "you already changed the past" effect; reality is mutable, whatever "reality" is.

There's something to be said for the concept of a universe with one mutable "timeline" existing with time travel inclusive in it being not really a timeline, nor even a universe with a singular timeline at all; instead, the timeline, which is really more of a set of points in spacetime and paths through thereof, is just a single iteration in a distinct progression of complete sets. These iterations are progressing through a causal stratum in an orderly fashion, with this higher causal stratum being a kind of "meta-time", where very little goes from one iteration to the next, and every iteration is causally complete, with the possible exceptions of what comes from the (immediately) preceding iteration(s). The alternative is that every iteration is not causally complete, and that if someone, say, goes back in time on 11:59:59 PM, December 31st, 1987 and changes the past, even a little, this not only spawns a new iteration, but the old one has no 1988 or onwards. (Adjust that for time zones, relativistic time distortion, the lack of absolute simultaneity, et cetera, where applicable.)

I'm really not sure if Destiny's time travel is one or the other, or if various systems within the Destiny universe function differently in that means, or what, though it's entirely possible that Destiny time travel isn't causally consistent, even on the highest level of abstraction. If the Darkness really did have acausal effects, it's a question of whether their cause did happen, but just got directed there through a strange shape in space and time, or the cause was brought in from another reality, or if the Darkness really was just transcendent enough to make something happen with no proper cause.

It also quite possibly plays by rules that don't model consistently (within individual systems, lack of consistency between various systems is fine) and/or at all coherently, but I'd rather not think that; it's less fun.

...Hoo boy, isn't this a doozy.