The X universe is a single-player space sim and a sandbox with a heavy emphasis on giving a player the freedom to take lots of different paths with a rather loose story line that you can play or ignore. The series started in 1999 and the newest entry in the X4: Foundations just launched yesterday.
I haven't played al of the games in the series, but I played X3: Albion Prelude pretty heavily and eventually built a pretty big commercial empire with my own space stations and a fleet of ships. Incidentally, I played it under Linux and the new X4 game uses Vulkan and will have a Linux launch coming soon (Windows only right now). This is the type of game that doesn't give you an elaborate central plot, but gives you the opportunity to trade and engage in combat mostly on your own terms, which is more to my personal liking since I don't need to feel like I'm in a scripted movie to enjoy a game.
I gave the most recent installment (X Rebirth) a miss since it took away many of the fun features like being able to pilot any ship from a fighter all the way up to a capital ship. However, it looks like Egosoft heard the complaints from users and X4 appears to share most of the positive features from the earlier games in the series while further improving graphics and making the gameplay more dynamic. Also, while there's certainly some post-launch polishing to do, the reviews I've seen on Youtube with live footage from yesterday's launch seem to show a game that's working pretty well with acceptable performance (Rebirth was notorious for not meeting those goals).
Something about these games: Playing for 5 to 10 hours is generally referred to as "just scratching the surface" since most games tend to mean you play for a long time!
Who needs Star Citizen!
Here are links to a couple of post-launch online review and play videos:https://www.youtube.com/watch?v=onW160fisG8https://www.youtube.com/watch?v=A4GQXz3zhschttps://www.youtube.com/watch?v=B2KrmYmRRBU