I'm new here.
Is there a word that defines "a thing that I/you don't know"?
Hello, why you just draw 'a thing that I don't know' on my avatar?
Thanks if answered it.
There is no single noun that means "something I don't know".
You could say
Draw something unfamiliar to me.
Draw something I've never seen before.
The adjective strange can mean "not like anything familiar (to me)", so you could ask:
Why did you draw that strange shape?
There is no single word that means a thing (I / you) don't know in an exclusive sense (where other people do know it).
The closest you're going to get is, appropriately enough, unknown:
: not known or not well-known
also : having an unknown value
// an unknown quantity
2 : something that requires discovery, identification, or clarification: such as
a : a symbol (such as x, y, or z) in a mathematical equation representing an unknown quantity
b : a specimen (as of bacteria or mixed chemicals) required to be identified as an exercise in appropriate laboratory techniques
// a disease of unknown cause
// Much remains unknown about his early life.
In your example sentence, you could say:
Hello, why did you draw something unknown on my avatar?
The use of the word in that context, however, is unusual. It implies that it's not only unknown to the person asking but also to the person who drew it.
Normally, you would have to add to me:
Hello, why did you draw something unknown to me on my avatar?
However, it's also possible that it could be used without any such qualification in a different conversation and if it is meant to apply to everyone:
"What's that symbol you drew on my avatar?"
"It's unknown. It was found on a wall a decade ago but it has yet to be interpreted."