Mass Effect’s Codex — an in-game encyclopaedia of lore accessible from the pause menu — contains entries that the game’s main storyline never references, covering subjects from dark energy physics to the quarian migration to pre-Reaper cycles of galactic civilisation.
The entry on dark energy, specifically, describes a cosmological phenomenon that becomes plot-critical in Mass Effect 3 — but only the version of the plot that BioWare originally planned and later abandoned. The Codex entry describing dark energy’s threat to the galaxy was the foundation for an ending in which the Reapers were attempting to prevent, not cause, extinction — using organic life as a counter to dark energy collapse.
This ending was replaced with the synthesis/control/destroy choice that shipped, reportedly due to development constraints. The dark energy Codex entry survived as a remnant of a different game — one that would have used worldbuilding planted in the original Mass Effect three games later.
The ME3 Codex adds a note to the dark energy entry acknowledging the phenomenon as observed but unresolved. BioWare never confirmed the cut ending, but the Codex archaeology is thorough.
This is the kind of discovery that keeps communities alive for years. Well documented.
Really fascinating breakdown — I had no idea this was hidden in plain sight the whole time. Going back for another playthrough immediately.