This is all that I know about the matter.
这句话主句是系表结构.
all是表语,that引导定语从句修饰all.
在定语从句中,先行词为all,everything,everyone等不定代词时,要用 that不用which来引导.
如果不用that,那么I know about the matter 就无法作为定语修饰all.
这是英语定语从句的语法规定,必须遵从.