(Bloomberg) -- Nasdaq Inc. agreed to buy off-exchange stock-trading venue LeveL Markets to expand its role in the equity market, according to people familiar with the matter. The move gives one of largest US exchange operators access to a growing segment of the equities market happening off-exchange. Alternative-trading systems offer an anonymous way to process trades outside of public exchange venues. Using off-exchange platforms helps institutional investors limit information leaking to the market and adversely affecting prices. Unlike public exchanges, dark pools aren't required to regularly disclose the same information about their trading systems.