People still live on Earth, and they send ambassador-governors to the colonies. The colonies interact a fair bit, mainly through the trading ships, but also things like universities and corporations. And the future-internet.

That may be the difference between European countries and their immigrant populations and the US: we're so used to immigrants, we (eventually) get over it and move on, and shop at their ethnic grocery/eat at their ethnic restaurant. Their right-wing nationalists and ours are similarly assholish, though.