There are so many things we don't know about in this world.
There are so many matters we really don't understand in our world.
この場合、次のような言い方ができますよ。
ーThere are so many things we don't know about in this world.
「この世にはわからないことがたくさんある」
ーThere are so many matters we really don't understand in our world.
「この世には本当にわからないことが多すぎる」
ここの「わからないこと」は
ーthings we don't know about
ーmatters we really don't understand
のように言えます。
ご参考まで!