Many sci-fi is fairly grim things. You will discover interplanetary wars. There are fascist societies with evil overlords. There is Loss of life and deception etc. But it surely’s not all doom and gloom. The proof would be the beloved (and often adapted) series of books beneath the blanket name https://fantasynovelsforteens99705.pointblog.net/Detailed-Notes-on-Fantasy-book-series-52251242