There's plenty of concern about bond markets. Prices are sky-high and trading is freezing up. But bond ETFs are roaring ahead.

In the first six months of 2016, some $67.6 billion in net new global assets flowed into fixed income exchange-traded funds, according to research firm ETFGI. This was almost double the $35.3 billion in net new assets these funds had gathered by the same point in 2015. Fixed income ETFs collected 55% of all net new money that moved into these passive investment products in the first part of 2016.