A look at Dark Pools: http://www.cnn.com/video/data/2.0/video/business/2014/09/26/cnn-orig-what-is-wall-street-dark-pool-paul-lamonica.cnn.html
and from Bloomberg:
“The biggest banks created the venues partly to avoid paying stock exchanges for processing trades between their clients. They’ve attracted scorn this year for allegedly letting the speediest traders feast on slower