I didn't know Japanese anime were so popular in other countries.
I had no idea Japanese anime were as popular as they are all over the world.
この場合、次のような言い方ができますよ。
ーI didn't know Japanese anime were so popular in other countries.
「他の国で日本のアニメがそんなに人気だと知らなかった」
ーI had no idea Japanese anime were as popular as they are all over the world.
「世界中で日本のアニメが人気だと知らなかった」
ご参考まで!