Equity prices from the New York Stock Exchange and Nasdaq flash across television screens, computers and mobile phones constantly. But there is a flourishing and hidden side to the business of buying and selling shares that most investors are not aware of.

The Securities and Exchange Commission is expected to expand on recent proposals governing this hidden side, so-called “dark pools”, which has grown in recent years. Last week the regulator started a 90-day consultation for suggestions on further proposals to reform the operations of dark pools.