The world's hottest investment product is showing signs of slowing down

Wikimedia Commons Exchange-traded-fund closures hit a record high for a second straight year in 2017, according to Citigroup data. The firm says that's just one signal the ETF market is maturing as traders shift how they're using the funds. It used to be that if you wanted to attract interest in an exchange-traded fund, all you had to do was launch one. After all, as the ETF market exploded over the past decade, investor demand seemed to outpace supply. That's no longer the case, which could mea...