Nothing. In the retail forex trading market, there is no tangible exchange ever. All FX trades exist virtually in a computer and are computed out per market price.
So why then does the FX market even exist? To simplify the day-to-day exchange of one currency into another for large organisations that continuously trade in the foreign market. More often than not, the main reason that FX markets exist is so that these multi-billion institutions can “express” their opinions on day-to-day fiscal and political events occurring in the world. This “trading” is, of course, purely speculative.