Dark pools are there from decades so it is nothing new. Also these are created initially to serve the purpose of institutions where there can be change of ownership of large chunk of shares with out much price distortion, similar to our Block deals in India, check this to know how block deals work in Inida.
Currently dark pools are price makers also so I am not sure if volumes are reported in live basis or with delay.
I believe they are popular only in US compared to any other individual country.