"Your younger brother is known by a lot of people."
Translation:Adikmu dikenal banyak orang.
It's a question I ask, too. For me, it makes sense: (i) if 'oleh' is implied - through a given context - here; it doesn't make sense to omit 'oleh' (ii) because what is the fuller context?
Looking at the relationship between 'dikenal [...] banyak orang', we see that many people know something; and they know 'your younger brother'. Therefore, reporting this to the '-mu' we get: "known by a lot of people". hence, why the 'oleh' might not be obligatory.