I seem to recall there being a 成语 to the effect of, "Heaven brought them together," or something like that. It expresses that the couple was truly meant for one another. Do you know which 成语 I am thinking of?
佳偶天成 (a perfect couple that's made by heaven)
天生一對 (born to be a perfect couple)
天造地設 (a perfect couple created by heaven)
天賜良緣 (a perfect marriage that was grant by the Heaven )
...which of these are perhaps more commonly used than the others?
佳偶天成, 天賜良緣, and 天作之合 are very literary phrases-- you usually find them in written form; and you only hear them commonly in wedding speeches.
天生一對 and 天造地設 can also be heard in wedding speeches, but they are more applicable in day-to-day conversations. They can describe something other than 'perfect couple'
天生一對 can refer to persons or things being extremely comparable or alike. For example: "你們倆一個是瘋子，一個是狂人, 真是天生一對." (one crazy, one mad; you two really are born to be a pair)
天造地設 can refer to persons or things being perfectly matched or perfectly suited in an arrangement. For example: "這是個天造地設的安排" (this is a perfect arrangement)