Bitcoin burst out of the shadows in 2017, seducing Wall Street and individual investors alike even though many still struggle to understand precisely what it is.The cryptocurrencys rise is also pushing regulators to consider taking action after years of simply urging caution.Bitcoin started the year at $1,000 per unit in January but by mid-December had shot to within striking distance of $20,000, a dizzying climb that stoked fears of a bubble even in financial circles used to speculation and volatility.Bitcoin remains a major gamble as it is very much an uncharted-waters asset, said Nigel Green of investment firm deVere Group. An asset that goes almost vertically up should typically raise alarm bells for investors.But he told AFP bitcoins rise demonstrated the strength of global demand for cryptocurrency.The digital currencys moment in the limelight began on December 10 with the first trading in bitcoin futures on a mainstream market in Chicago.This is the year bitcoin and the cryptocurrencies went legitimate, said Timothy Enneking of Crypto Asset Management.With a peer-to-peer payment system based on blockchain technology, bitcoin is establishing a niche for itself: In some towns, consumers can now dine in restaurants and buy cars and even houses using bitcoin.