NEW YORK ( TheStreet) -- As evidenced by the National Stock Exchange's monthly fund flow data, the month of April proved to be another popular one for ETF investors. Since ETF assets initially broke through the $1 trillion mark at the start of 2011, the universe has continued to grow and expand at an impressive stride. At the close of April, total assets stood on the cusp of breaking through $1.12 trillion.Additionally, the industry welcomed the addition of 23 new funds. This lifted the universe's total product count to 1,030. On top of market appreciation, strong investor inflows played a major role in boosting assets. During the opening weeks of the second quarter of 2011, the universe saw net inflows totaling more than $21 million. This represents the largest single-month inflow of the year. Industry leaders BlackRock ( BLK), SSgA, PowerShares and Vanguard led inflows, with $8.9 billion, $4.9 billion, $2.8 billion, and $2.8 billion entering, respectively. On the other side of the coin, U.S. Commodity Funds and Direxion lead the industry in outflows, respectively witnessing $820 million and $575 million head for the exits. Fixed-income giant, Pimco, was another sponsor that witnessed notable outflows. During April, more than $290 million left the firm's products. The bulk of Pimco's losses can be traced back to the outflows witnessed from the fund's active product, Pimco Enhanced Short Maturity Strategy Fund ( MINT). The fund saw the universe's fourth-largest outflows, $311 million. While MINT's decline was heavy, it was dwarfed by the outflows witnessed in the United States Oil Fund ( USO). Despite oil's prolonged strength, investors appeared to have used April as a time to pocket gains. During the month, over $680 million flowed out of USO. Fellow energy-related ETFs were not immune to outflows either. Energy Select Sector SPDR ( XLE), SPDR S&P Oil & Gas Exploration & Production ETF ( XOP), and fellow futures-based fund, United States Natural Gas Fund ( UNG) saw heavy outflows as well, totaling $670 million, $310 million, and $187 million, respectively. In the face of silver's dramatic run up, the massive iShares Silver Trust ( SLV) was among the list of outflow leaders, with more than $170 million exiting. Interestingly, other silver funds, including fellow bullion-backed ETFs Physical Silver Shares ( SIVR), saw inflows. This divergence may be attributable to SIVR's considerably lower expenses. While SLV charges investors 0.50%, SIVR carries a 0.30% expense ratio.