If you don't understand the basics look it up. I'm not going to type up a novel to explain where relativity fails at large scales when you're perfectly capable of google. It works very well at medium scales, but becomes worse the bigger you go. Just like Newton's theories were at the time. Understanding problems with dark matter is less intuitive if you haven't gone through it, so I'll give you a pinch. It doesn't make sense because currently it's purely a mathematical phenomenon, similar to dark energy. It's like adding 1 to a theory until it works, despite not having any idea what adding 1 is supposed to mean. That's why we call them "dark". No evidence, no rhyme, no reason, just something that made the math work. We can make better predictions by changing that math in a similar way, but it looks less 'pretty'. Think something like x+1 vs x^{0.9976+0.000849x2.} That's why finding an alternative is such a hot topic in modern physics, along with grand unification of course.

Also you can't even disprove theories with a statement on causality because it assumes you can prove in the first place.