everything后是不是省略了一个关系代词that呀？

They've done everything they can possibly think of
凡是能想到的，他们都去做了。
have done 是谓语动词，现在完成时吧？
everything they can possibly think of：everything后是不是省略了一个关系代词that呀？that作宾语时可以省略的。

have done 是谓语动词，也是用的现在完成时形式。
everything是不定代词，后面的定语要用that修饰，that在从句中做宾语时可以省略。

应该不是吧。