Argthtjtr

Help me, is there any English words that means "thing that i/you don't know"?

Thanks if you answered it.

Per Donald Rumsfeld:

There are things we know that we know. There are known unknowns.
That is to say there are things that we now know we don't know. But
there are also unknown unknowns. There are things we do not know
we don't know.

One word I know of is ignorant.

One of the definitions is essentially: lacking knowledge or awareness in general

So, I could say that I am ignorant on the subject of building a space rocket or that you are ignorant on the subject.

Although it is often used derogatorily, it technically just means you don't know about something.

An interesting synonym for ignorant is nescience, which as I understand it means the same thing, but doesn't sound as rude.

Edit #1:

After reading the comments, I realize that I am ignorant as to what you are asking exactly. Using your example sentence, I think a lot of people end up using vague words like thing or stuff to reference something difficult to name exactly (https://dictionary.cambridge.org/us/grammar/british-grammar/uncountable-nouns/thing-and-stuff). Or we might refer to an unknown as this or that simply because we may not have the right word at the moment to describe it.

For example: "What is that thing over there? It is making a funny noise."