に違いない
Usage
This structure indicates that the speaker is certain that something is true or is the case. Must, I'm sure/certain that; It is certain that
Examples
スミスさんは日本に１０年も住んでいたから、日本語が上手に話せるに違いない。
Smith must be living for ten years in Japan, because his Japanese is very good.
あの二人はとても仲が良かったから、卒業した後、結婚したに違いない。
That two people have a very good relationship. After they graduate I am sure that they will marry.