They lost their way in the forest,and ____(what)ma

Question

They lost their way in the forest,and ____(what)made matter worse was that night began to fall.请问空格那里为什么填what呢？

Answer 1

what 在这里引导主语从句 what made matter worse在这里做主语，表示“让事情更糟糕的是...”，what引导主语从句的时候翻译成“......的”