In the ME2 DLC Arrival, the Reapers are revealed to be entering the galaxy near a specific Relay so that they can use that relay to get onto the network and start attacking. After we blow it up, they are then stuck at the edge of the Galaxy, proceeding much more slowly.

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> She says that she and her people were investigating rumors of Reaper artifacts on the fringes of the system. Kenson claims they found proof that the Reapers will arrive in the system and when they get here, they’ll use the mass relay to travel throughout the galaxy. Her team calls it the Alpha Relay and from there the Reapers can invade anywhere in the galaxy. [Source](http://masseffect.wikia.com/wiki/Arrival_%28assignment%29)
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Shepard then deduces that, given the story, Kenson chose to destroy it. She confirms that. It would stop the Reapers' invasion as it would take months or even years at standard FTL speeds to reach another relay

However, at that point, there is a massive Reaper fleet, in the galaxy. How is it possible that no one knows this? Additionally, how could we lose contact with the majority of the Solar system without anyone getting any signals off?

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> "We just lost contact with Luna base"
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Wouldn't the lunar base have some sort of long range tracking, and tell Earth command?

To summarize, since the official canon status at the end of ME2 is that the Reapers are in the galaxy and the Alliance knows about it, how could they possibly be attacked and not know about it, with all communication just stopping totally?