There is ample evidence that dark matter exists, as in, there is something there that we can indirectly observe. We have fairly good ideas about what it isn't, we just don't know what it is for certain.

Dark matter (LCDM) solves some problems and creates others, just like modified gravity theories solve some problems and create others. Hopefully more experimental data will lead to an answer.

Or the models and maths fail at a certain threshold.