"Exchanges" seem to have a somewhat biased view of trading, maybe, as looking at things like this thesis:
fiximulator.org/FIXimulator_Thesis.pdf
and looking at the whole "marketcetera" platform, and at the FIX protocol for equjities trading, and such sources, it reads a lot more like a vast world wide distributed network of traders, with to some extent a "buy side" where fund managers and such see things from their side, and a "sell side" where middlepeople such as stock exchanges try to find buy side folk looking to buy different things and match them up, partly presumably by means of the stuff some buyers are using as wealth to buy stuff with happens to be stuff that other folk are actually looking to buy.
Or something.
Certainly it is not "centralised", heck not only do some nations have more than one stock exchange but there are more and more nations that have at least one stock exchange.
Some of this stuff makes it seem like the so called stock exchanges are just gaggles mobs of sell-side folk ganging up the better to out-do the smaller sell-side folk..
fiximulator.org/FIXimulator_Thesis.pdf
http://www.marketcetera.org/http://www.fixprotocol.org/-MarkM-