Creating hundreds of levels at runtime

I am working on a space game; When the game starts, it will generate a galaxy of 100+ systems. Each system will have randomized planets, asteroid fields, etc. The galaxy is like a map that lets the player travel between systems, while each system is the actual level where you fly your ship.

How would I handle 100+ levels where their content is generated at runtime? If they are all separate levels, is there a way to make a new level during runtime or should I save all the level data somehow and load in all the actors in one level when a player “travels” to a different system?