Eh, I'm fine with waiting until we get an official explanation of how the intergalactic travel will happen. Right now, this thread is pretty much just an exercise in lots of "Waltersian Speculations."
If it's a wormhole, then cool, I hope they write the story well enough to sell me on it. If it's something other than that, then cool, I hope they write the story well enough to sell me on it.
Doesn't have to be a wormhole, you know. Why not (as a few have mentioned) the Citadel itself? According to ME1, it's also a huge mass relay that leads directly to some point in dark space where the Reapers usually hibernate. Given that Andromeda is 2.5 million light years away, then the dark space relay that the Citadel connects to could be one or two million light years away. The Citadel relay's got to connect to an opposing relay somewhere, right?
Perhaps it connects to one in dark space that is part of a hub of other dark space relays which connect to other galaxies millions of light years away? That would assume that Reapers have gone to other galaxies, though, but I might be able to buy that story.
It's also interesting to note that so many people here are quibbling about how believable a plot point a wormhole or some other "space magic" would be, yet everyone who has played the game seems totally fine with overlooking the fact that (just like most science fiction stories), the Mass Effect universe totally ignores the effects of time dilation. Special relativity tells us that by the time Shepard and crew had used their FTL drives and Mass Relays to zap back to Earth, the actual "battle for Earth" would've been over for decades, if not centuries.
If we can overlook that, we should be able to overlook/accept a newly discovered wormhole or some such other space magic. My money's on using the Citadel relay to dark space, though.
It's fun to debate this but I hope no one is taking it too seriously just yet. So far, we have no idea what the story is or how they set it up. We just have rumors from a supposed survey whose authenticity is in question.
And, we have "lots of speculations."
Somewhere, Mac Walters is reading this thread in a dark room, chugging a Molsen's, smiling with pride as a "speculative" tear runs down his cheek.
Edited to add: Great minds think alike. I was typing this at the same time ZipZap and Hanako were posting pretty much the same things. 