Mamma mia !

it means = oh my God!

Oh my god

I mean u can say 我不知道了 but it is a little weird........

我的妈呀！== Oh my god!!!!!! PS: plz say “我不知道" not “我不知道了” we can say "我知道了" but we dont say "我不知道了" however we say "这我就不知道了" (for this, I really dont know) 就 and 了 are used for highlight "dont know about 这(this)"