To my mind, space sci-fi represents the optimism for the future that has yet to be repressed by people's increasingly negative outlook on our civilization's future. They are futures full of exploration, danger, and wonder, with cultures both different and the same. While many space sci-fi are dark in atmosphere, that doesn't get around the fact that even in the darkest space science-fiction, there are wonders and horrors that stimulate the mind and the spirit.
I usually don't make distinctions between types of future sci-fi (as opposed to modern day sci-fi), incidentally.
Space sci-fi is relatively rare in JVNs, which are my specialty. One that comes to mind is Fake Azure Arcology, where people were forced to flee from a massive storm of asteroids that devastated Earth's surface, forming unique civilizations within the arcologies they fled to. In a mix of high technology and restrictive lifestyle, it describes the unchanging human condition, even in circumstances that are drastically different.
Komorebi no Nostalgica, one of my favorite sci-fi VNs of all time, is based in a world where humans warred against their AI slaves until the slaves won freedom, at the cost of taking the bullet for humanity destroying its own history for the convenience of politicians. It is a world where, interestingly, history is mostly lost to the younger generation, with the older generation only having faded memories of what was destroyed. In this game, society has almost become a blank slate, and the game extensively explores the troubles and possibilities of human-like AI in the two different types - the ancient android Cinema,
, and the victorious Metocell, whose emotional emulation evolves over time and experience.
More familiar to English-speaking readers is the dystopian worldscape of the Baldr series, where people have practically fled into the virtual world to escape the limitations of the physical. While the Baldr series is a fun read, it is also the most cynical and pessimistic approach to sci-fi I've seen in VNs. It shows the nastiness of human nature, the poor decision making of those with power, and the inevitable failures of a society that lost its way long ago again and again. In the Baldr series, AI alternates between 'ghosts' of the deceased, god-like beings that are omniscient in their own way, and monstrous amalgams that blindly follow their programming with only a surface imitation of flexibility and thought.
Sci-fi can be a wonder or horror, but it is always looking at the future.