After the Silk Road online black market was shut down, several sites competed to take its place. Evolution became the new king based on its professional appearance and seemingly secure payment system. It appears that system wasn't very secure though because the site has vanished and $12 million in users bitcoin is gone as well. 

People claiming to be staff members say that the pseudonymous founders of Evolution, "Kimble" and "Verto," intentionally scammed users, delayed funds transfers for a week and made off with the accumulated bitcoins. 

And so the bitcoin/darknet world turns.