It's not so much an asteroid belt as its a ship wreckage/graveyard. The earthbound Awoken are a pretty interesting issue, on the one hand, it's impossible from the perspective of the story that they didn't come from the reef, which is seemingly a dubious task considering how the reef was made from broken apart ships. Another issue, is that most Awoken consider themselves to be neither affiliated with the light or dark, yet it's clearly stated multiple times that your Guardian is "Forged in Light" from the moment of its creation, technically making earthly Awoken, not Awoken. Either way, there was too much confusion and not enough information on the subject to find a spot in the timeline for this to have occurred.