I have played in 3 different systems, and I have heard everyone talking about smashing planets with asteroids, but there are no asteroids anywhere. How do I get them into a system?
Currently asteroids do not exist in the auto-generated systems. You have to manually add moons and scale them down in the system editor. It's a confirmed feature that there will be a way to automatically create asteroid belts without having to manually place them, but it's not implemented yet.
Welcome to the forums! Traditional astroids are not in the game. Right now you have to use small moons for the same purpose. Randomly generated systems automatically created in matchmaking are only 2 planet systems. Go to the system editor and build a custom system and use that.