problem

i have a friend who recently seems more and more like depressed .. not suicidal depression but like declining humor and stuff .. and they have always been kind of secretive and what not like i have known this person for a long time now and yet i know so little about them at the same time.. and tbh i dont know what i can do to make them feel better .. in reality i have looked at possible scenarios but im not able to be the person that they can come to to talk to about that stuff .. and i really want to help but i can't.. i can only theories about the problems but i know nothing and it sucks.. idk if anyone has any suggestions on what to do please let me know