Complex sentence: Preis did this instalation with very little or no publicity, so his instalation isn't well known among citizens of Zagreb, so in few occasions individuals or small groups of people "discovered" that there's a model of solar system in Zagreb.
Split into simpler sentences: Preis did this installation with very little or no publicity, so his installation isn't well known among citizens of Zagreb. On a few occasions individuals or small groups of people "discovered" that there was a model of the solar system in Zagreb.