More recently, Wall Street wiseguys have shifted from buying loans to making them directly – in this case to midsize companies, long considered the last preserve of the traditional banking system. The new players are private equity firms, hedge funds, investment banks, insurers and once obscure entities known as “business development companies.”

In each of the last four years, investors poured more than $100 billion into these middle-market private credit funds, which now have combined assets of between $600 billion and $900 billion, according to estimates by Bloomberg News; Preqin, a data company; and Ares Capital, a leading direct lender. The banks’ share of middle-market lending has been nearly cut in half as private lenders have not only taken business away from the banks but also significantly expanded the market by making loans that banks are unwilling or unable to make.

“It’s a wild west space,” a top credit strategist from Bank of America told the Financial Times this year. “The whole thing has exploded in size, and everyone is getting into it.”