While one can just stumble over a guardian during exploration, they can also be found with the help of the Curator Enclave.
They are marked with a skull icon both in the fleet power box and on the galactic map, and the exact strength number can only be seen with debug mode on. They can be found in special predetermined systems and rival moons in size and mass.
Guardians are extremely powerful Spaceborne aliens added in various DLCs.
