Elite Dangerous has been around for a while now, multiple years in fact. Ever since the game's release, all life in the game was human. For all the player base knew, humans were all there was in the galaxy aside from relics and unknown artifacts. Then this happened.
No update announcement, no "Go search for a secret we hid", no teasing from the developers. Alien life was absolutely kept a secret within the development team at Frontier Developments. This is what made first contact with Xbox One player DP Sayre so monumental. They discovered aliens. We can't even imagine the surprise upon their reveal.
It wasn't until this video was posted that Frontier Developments finally officially revealed the alien race as Thargoids. What's even eerier is the fact that all we have is a name and a look at their ship. Nothing else has been revealed.
Elite Dangerous is a relatively popular space sim, with a niche but dedicated audience. It's similar to No Man's Sky but arguably takes a slower pace and differs in gameplay. Its main focus is space exploration as opposed to planetary. This kind of reveal is something many games today don't or wouldn't do, as the almighty sales numbers rule all, but with Elite Dangerous's release strategy of huge content dumps over time, having a secret planned to be discovered in a game about discovery works, and it did, in fact, work flawlessly.
Better get back out there and see what else is in store.