뭔가 means 'something, without saying what', and with 알다 it describes a sense of knowing you cannot quite pin down: 나 뭔가 알 것 같아 (I think I'm onto something), 뭔가 아는 눈치 (he looks like he knows something). The negative 뭔가 알 수 없는 means 'some indefinable ~', as in 뭔가 알 수 없는 울분, and 알게 모르게 is a set phrase for something happening without anyone quite noticing.