What is the current state of the hypothesis of mirror matter today? Are there any experimental data or theoretical arguments that exclude it by now, or is it still considered viable among physicists?
The relationship between mirror matter and ordinary matter is different from matter-antimatter which are related by space-time reflection.
As far as I can make out Matter-Mirror matter is hypothesised to be related by space reflection alone.
Mirror matter theorists like Dr.Robert Foot believe that dark matter might be made up of mirror hydrogen and mirror helium.