Large institutional investors sometimes trade on “Electronic Trading Crossing Networks," which allow them to conduct trades without publicly exposing them.
They are used by financial institutions to move large blocks of shares without public investors even knowing about such transactions. Such examples of networks are “Liquidnet,” “Pipeline,” “SIGMA X,” and many others.
It might be difficult to fathom the size of the transactions conducted over these networks, but the ownership of dark pools involves almost every institutional trading house. This is a huge business and regulators are carefully looking into their activities.