In that context, I'd actually prefer them to stick with a human-only universe.
Hm. I'm trying to think -- how many space-based SFTV series have there been set in universes where humans have spread across space but never met aliens? There's
Red Dwarf (where all the nonhuman species were genetic constructs created by humans, or descended from an Earthly species in the case of the Cat),
Firefly (although that's borderline since humanity had only traveled to one system post-Earth), the
Battlestar Galactica/Caprica universe (where "life here began out there" but humans and their Cylon creations were the only sophonts), and now
Killjoys and
Dark Matter, apparently. I would've thought there were more, but TV Tropes's
index for the trope lists no other television examples, although there are surprisingly many prose examples.