Bitcoin and cryptocurrencies like it that don't exist outside cyberspace and have suffered rapid price fluctuations represent "the mother of all scams," says Nouriel Roubini, the New York University economist who predicted the 2008 financial crisis and was nicknamed 'Dr. Doom.'

"Literally, every human being I met between Thanksgiving and Christmas of 2017 asked me" if they should buy cryptocurrency, Roubini said in written testimony prepared for a Thursday hearing before the Senate Banking Committee.

Cryptocurrencies were exploding in value at the time, with Bitcoin alone peaking at $19,343 in December, compared with $1,000 earlier in the year, and their backers "tapped into clueless retail investors' FOMO — fear of missing out — and took them for a ride," he said. Bitcoin has since tumbled 66 percent to $6,555, according to Coindesk.

As a result, cryptocurrencies have drawn the attention of Trump administration officials from Treasury Secretary Steve Mnuchin to Securities and Exchange Commission Chairman Jay Clayton, who worried that investors didn't understand what they they were buying and that the products could undermine financial stability.

Those are among the issues that prompted corporate titans from Warren Buffett to JPMorgan's Jamie Dimon to turn up their noses at the prospect of doing business in cryptocurrency and spurred the Senate Banking Committee to schedule a hearing with Clayton and Commodity Futures Trading Commission Chairman J. Christopher Giancarlo earlier this year.

Cryptocurrencies are generated, or mined, when networked computers record and validate transactions in blockchain, a secure ledger accessible worldwide by multiple parties. Buying the equipment to handle mining, however, can be costly in itself.

"Experience tells us that while some market participants may make fortunes, the risks to all investors are high," Giancarlo and Clayton wrote in a column for the Wall Street Journal. "These markets are new, evolving and international. As such they require us to be nimble and forward-looking."

Trading in cryptocurrencies was banned last year by regulators in China who pointed to concerns about maintaining "economic and financial order," and JPMorgan's Dimon has said other countries are likely to do the same.

Establishing a sovereign currency is one of the first actions governments undertake when they're formed, and none are likely to stand by while a competitive money supply they don't control grows large enough to become a threat, he said.

For money to have value, only so many currencies can operate alongside each other in the same market, Roubini said. Otherwise, it becomes increasingly difficult, if not impossible for consumers to determine the value of goods and services.

"In the U.S., the reason we do not use euros or yen in addition to dollars is obvious: Doing so would be pointless, and it would make the economy far less efficient," the economist said.

The idea that hundreds of cryptocurrencies could viably operate together "contradicts the very concept of money," Roubini added. "Their supply is created and debased every day by pure fiat and in the most arbitrary way. So cryptocurrencies are creating crypto money supply and debasing it at a much faster pace than any major central bank ever has."