Once Nixon abolished the gold standard, the dollar's value quickly plummeted. The dollar index was established to give companies the ability to hedge this risk. Someone created the U.S. Dollar Index to give them a tradeable platform. Soon, banks, hedge funds, and some speculative traders entered the market. They were more interested in chasing profit than in hedging risks.
For instance, if we take a less active period between 5 pm – 7 pm EST, after New York closes and before Tokyo opens, Sydney will be open for trading but with more modest activity than the three major sessions (London, US, Tokyo). Consequently, less activity means less financial opportunity. If you want to trade currency pairs like EUR/USD, GBP/USD or USD/CHF you will find more activity between 8 am – 12 am when both Europe and the United States are active.
The foreign exchange market (Forex, FX, or currency market) is a global decentralized or over-the-counter (OTC) market for the trading of currencies. This market determines the foreign exchange rate. It includes all aspects of buying, selling and exchanging currencies at current or determined prices. In terms of trading volume, it is by far the largest market in the world, followed by the Credit market.
According to the Bank for International Settlements, the preliminary global results from the 2016 Triennial Central Bank Survey of Foreign Exchange and OTC Derivatives Markets Activity show that trading in foreign exchange markets averaged $5.09 trillion per day in April 2016. This is down from $5.4 trillion in April 2013 but up from $4.0 trillion in April 2010. Measured by value, foreign exchange swaps were traded more than any other instrument in April 2016, at $2.4 trillion per day, followed by spot trading at $1.7 trillion.