The amount of money raised by private equity firms but not yet invested – known as dry powder – hit a record high of $1.073 trillion globally at the end of 2013, according to data provider Preqin, an increase of $130 billion from 2012. The total has continued to grow, reaching $1.141 trillion globally as of the start of June.

"Private equity has been the best-performing asset class for many institutional investors over the long term," says Hugh MacArthur, head of global private equity at consulting firm Bain & Co. "In a world where many other investments, like credit-based investments, offer a very low yield, they're saying they need to continue to pursue those returns, so they're putting more money into private equity."