The revenue Robinhood gets from a controversial practice of selling customer trades to high frequency trading firms is skyrocketing, according to new research.

Robinhood has faced criticism over its reliance on high-frequency traders, especially considering a founding ethos that some have categorized as “Anti-Wall Street.” The company sends customers’ orders to high-frequency trading firms like Virtu or Citadel Securities instead of a stock exchange like the NYSE. These trades are executed in what’s known as a dark pool, which as the name suggests, lacks some transparency.

Robinhood makes money by taking a fraction of a cent per dollar from each trade order and collecting interest on customers’ deposits.

As a private company, Robinhood is not required to disclose its income statements, which would paint a better picture of its revenue components.

According to a Bloomberg report last year, Robinhood brought in more than 40 percent of its revenue in early 2018 from selling its customers’ orders to high-frequency trading firms, or market makers.

Shortly after the report, Robinhood’s co-founder and CEO Vlad Tenev defended the practice in a blog post.

The growth in Robinhood’s revenue came alongside a massive increase in customers.