There are references to Earth in many Star Wars comics and games, but it cannot be reached by any civilization due to the Hyperspace anomalies surrounding the Galaxy. These 'disturbances' were placed there by the Celestials, an incredible powerful alien race who quarantined an ancient alien Empire from expanding beyond the Galaxy.
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E.T. Got through.
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... That's a vague and non-canonical link. Also, it's possible to ENTER the Galaxy, through Vector Prime, but it's nearly impossible to leave.
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Oh ok, can't you enter through that strange mini Galaxy just outside of the ring? Also if it's so hard to get in, then how would the halo universe get there?
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They have slipspace tech, and Star Roads. They don't work like hyperspace does.