Bond ETFs listed around the world totaled $500.3 billion in assets under management by the close of trade on Thursday, according to BlackRock, the largest issuer of such funds, still only a fraction of the $100 trillion global debt market.

Most ETFs are held in funds listed and traded in the United States, where investors bought more than $43 billion in the year through September. Government debt funds and short-maturity bonds were particular favorites as they are relatively resilient in a rising interest-rate environment rise.

ETFs, which debuted in the United States in 2002, are increasingly used by both retail investors and institutions as a replacement for corporate bond and government-issued debt.