In 2015, the US dollar dominated the global currency market, and the rise of Bitcoin was amazing.

The overall performance of the U.S. dollar was strong in 2015. With the Federal Reserve expected to raise interest rates, the trend of the U.S. dollar has attracted widespread attention from the global market.

In the past few years, the US dollar index continued to be weak due to the dovish tone of the Federal Reserve; in 2015, the US dollar index strengthened significantly, rising by nearly 9% against a basket of major currencies. The US dollar rose sharply against some currencies, such as the Brazilian real, bringing huge profits to investors.

However, when searching for the best global currency for 2015, investors should place a big question mark around the term "global currency."

Bitcoin has volatile recent years, soaring above $1,000 in some transactions before collapsing in late 2013, but has rebounded again this year, rising more than 40 percent to $428. Bitcoin supporters say the controversial digital currency will continue to rise despite controversy and a lackluster 2013-14 as the number of users and global transactions surge.

“Bitcoin is going up, and transactions are increasing,” said Adam White, vice president of Coinbase. “As it gets used more and more, its utility value will go up.”

So why has Bitcoin risen so quickly since October? During these three months, Bitcoin has gained most of its gains for the year.

Bitcoin supporters point out that while it is difficult to explain the direct causes of large price fluctuations in a short period of time, there are many factors that can help explain it, but none of them are "provable."

Just as Bitcoin began to soar in October, the European Court of Justice ruled in late October that Bitcoin transactions are not subject to value-added tax (VAT).

Additionally, bitcoin has seen a notable premium in China in recent months, highlighting high demand.