It has been clear for a while that exchange-traded funds would eventually break through $2 trillion in assets in the US. But it now looks like it will happen much sooner than most observers expected.

Backed by a relentless stream of new money and supported by a five-year rally in stocks-broken only by occasional volatility-assets in US-listed exchange-traded products are rapidly approaching the $2 trillion milestone and likely will pass it by early next year.