There can't be anyone out there who missed this reference, but it is our duty to include it anyway. This episode opens up with an establishing shot of space as the Doctor's voiceover says: "Space: the final frontier..."
This is, of course, a nod to that other long-running sci-fi TV institution: Star Trek. Specifically, the Original Series and The Next Generation. Every episode would begin with that iconic opening monologue that I'm sure we don't need to recite here.
This is far from the first time that Doctor Who has referenced Star Trek in the modern series. These include Rose nicknaming the Ninth Doctor 'Mr Spock' in The Doctor Dances, the Tenth Doctor making a Vulcan salute in Fear Her and the Silence being compared to Star Trek aliens in The Impossible Astronaut.