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It's referred to as the 'Wild West' of the Internet: Underground websites—often called the "dark web"—that sell illegal drugs, guns and stolen credit cards.

The sites have names like Evolution, Agora and Nucleus, where criminals can buy and sell all manner of illegal goods and services. The sites can attract hundreds of thousands of users, and pull in millions in monthly sales.

Law enforcement is working hard to close down these sites. Just last month, a joint global operation involving the FBI shut down dozens of these so-called dark markets.

But despite these efforts by the FBI and others, the underground economy is very much thriving, according to a report issued Thursday by the Digital Citizens Alliance.

"It took a major blow," said Dan Palumbo, research director of Digital Citizens Alliance, speaking of the FBI's crackdown on this online black market. "But we have seen that it is growing, and that it will continue to grow. There will continue to be large players in this space, and there will be new sites popping up offering the same drugs and services."

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Right now, there are still some 52,000 items for sale on these websites: everything from cocaine to fake passports, according to Palumbo.

One site in particular--called Evolution--is enjoying explosive growth. There are 27,000 items for sale on that site with a large section devoted just to credit card fraud and identity theft. Analysts say Evolution also attracts users because it's very easy to use.

It's unclear who operates these sites, but it is obvious that they can make a lot of money. One site called "Silk Road 2" appears to have generated $6 million in monthly sales before it was shut down.

Often, it's hard for law enforcement to close down these sites because technology hides them. Users access dark markets by first downloading and installing a browser called Tor, which masks their identities, allowing users to search in secret.

Dollars or euros are often exchanged for bitcoin, which are then used to make purchases on the sites. The digital currency is attractive because it's convenient, and harder to track than traditional payment methods.

"If you use a credit card then you might as well give your home address to the police," said Nicolas Christin, an assistant Research Professor of computer engineering at Carnegie Mellon University, who studies this market.

So, what's the future of this dark industry? Analysts say it will only get darker—and bigger.

"Now you see sites taking a different tack," said Palumbo. "They don't care about fraud. It could be a slippery slope into other things like allowing child pornography or murder for hire. I wouldn't expect that anytime soon, but I could see that progressing to where it's available on these websites."

And, despite the government's best efforts to destroy this market, analysts say it remains to be seen what kind of long-term impact Uncle Sam is really having.