User blog comment:Kaloneous/Mass Effect 3 Ending DLC Denied and PAX/@comment-4043223-20120329161240/@comment-2076032-20120329163912

If you're talking about when the ME3 ending was established, everything. Except I posted the wrong link, but this article explains it well. Karpyshyn was throwing around numerous ideas for the end of the series - the dark energy plot was one of them. So was the ending we got. These ideas existed back when they did ME1.

In other words, this is what they were planning to do all along. Or, at least, ONE OF the ideas they were planning to do all along.