The rest is in the form of deposits, which, most of the time, can be accessed electronically. So the vast majority of what we know today as money is a liability of the private sector, and not the central bank, and is already electronic. With most money available electronically, there has been a substantial shift to electronic forms of payments as well.
As a great inventor once said: “…if we did all the things we are capable of doing, we would literally astonish ourselves.”2  1  Meyer, Laurence H. (2002) “A Term at the Fed, An Insider’s View,” Harper Business. 2  Thomas Alba Edison.