A "theorem" is a proved proposition, so there is no such thing as a theorem that might well be unprovable.

The proposition you refer to (which is not a theorem since no proof is known, and in fact it has been shown that this proposition is logically independent of the usual foundations of set theory, so it cannot be proved in that framework) is called the Continuum Hypothesis:

https://en.wikipedia.org/wiki/Continuum_hypothesis