mass-effect-3
This nebula has been identified on side quests (I put all priority missions on standby … ironically) so that I don't miss out on any content. Is this a case of a galaxy just not being available until the main plot line catches up?
How far do I have to progress to open these up?
It doesn't show you exact numbers, but if you go to the squad menu and view your character, next to your current level it will show you a progress bar of how far along you are. It seems the best that you can do is see a number come in, then flip over to see how much your bar jumped. You can use this as a gauge for how close you're getting
Ultimately, you're better off just ignoring the actual numbers and simply enjoying the ride. Since there really aren't any mechanics in place to grind experience points, the actual numbers end up not mattering much.
Toward the end of the game at least, the functional fuel depots seem to mostly be concentrated in the lower left-hand corner of the Galaxy Map. In the following image, systems with fuel depots accessible directly from Mass Relays are circled in green (six total):
As you may already know, most systems will also contain fuel depot wreckage sites that you can scan for, for a one-time fuel bonus. However, if you're just looking to quickly top off your tank, visiting one of the above-circled systems is the way to go.
In particular, I believe Annos Basin (to the left of the selection cursor on the map) is available to the player for most of the game, and is never marked as being Reaper controlled.
Best Answer
The two IGN articles on them say that you gain access to them after the Priority: Thessia mission near the end of the game, so yes it looks like you need to progress through the main plot line to get them.