对!
You discover facts that show (that可省略) it is definitely true.
即：You discover facts that show it is definitely true.
第一个that指代facts,引导后置定语从句.
第二个that引导show后面的宾语从句.
翻译：你们发现了真相,这个真相表明了,某件事是完全对的.从句套从句！这样的现象一般是书面语吧，见得不多啊嗯，一般直接省略第二个that