Dark space

Dark space (or intergalactic space in real-world astronomy terms) is the name for the physical space between galaxies. Dark space is empty of stars, hence the name. Because thousands of light-years lie between any two galaxies (with a single light-year being exactly 9,460,730,472,580.8 kilometers) dark space is essentially a vast expanse of near-vacuum.

According to Vigil, the Reapers go into long states of hibernation to conserve energy. However, in this state they are vulnerable. By taking refuge in dark space beyond the galaxy's horizon, the Reapers ensure they will not be discovered by accident and destroyed while they wait for their vanguard to open the Citadel mass relay, allowing them to return to the heart of the galaxy.

At the end of the first game, whoever is chosen to be the human representative on the Citadel Council, either Donnel Udina or David Anderson, swears to lead the galaxy against the Reapers and "drive them back into dark space".

At the end of the second game, the Reaper fleet is seen out in dark space and appears to be emerging from hibernation and moving towards the Milky Way.