Steamunlocked: Space Engine

The developer allows the old version (0.980) to be distributed for free on their official website. It lacks the new nebulae and ships, but it features the entire 1:1 scale universe. It is 100% safe and 100% legal.

Space Engine frequently goes on sale.

If you are a student, email the developers. Many indie devs (including the Space Engine team) have been known to give out free keys to verified students writing papers or creating educational content. Do not steal; ask. space engine steamunlocked

If you just want gravity sims, there are legal open-source options: The developer allows the old version (0

Space Engine is not a static executable. It relies heavily on: Space Engine is not a static executable