Translation:There are many galaxies in the universe.
Zonnestelsel - Solar system = A star and things (e.g. planets, asteroids, etc) that orbit it.
Sterrenstelsel - Star system (A term Duo still refuses to acknowledge, despite the fact that it's more accurate and the LITERAL translation) = A group of stars (some of which may be part of solar systems) that orbit each other.
Galaxy - a star system containing a large number of stars, sometimes called a star cluster.
As far as I can tell, in Dutch, there isn't any disambiguation between "star system" and "galaxy"... I suspect this is because most scientific papers by Dutch people would be written in English.
This is a problem for astronomers in English as well as Dutch. The issue arose because when the term "solar system" was invented, there was only one known, and that remained true for more than a century, although others were assumed to exist, and finally discovered in the 1990s. So there was little need to differentiate between "the Solar System" meaning the one around Sol, our sun, and "a solar system" orbiting another star. In English, too, a "star system" is ambiguous, and can mean a system of stars i.e. a galaxy, or a system of planets around a star i.e. a solar system. I don't know how usage in Dutch has settled, but it seems like Duo is trying to get us to use zonnestelsel for "solar system" even when the star in the middle is not de zon.