Along with the S&P’s rise has come a surge in ETF assets.

The U.S. industry boasted just shy of $1 trillion in 2010 and today holds nearly $3.1 trillion, according to Investors have chosen passive strategies amid the low volatility and with central banks as a backstop in case instability creeps back in.

Singer isn’t the only big market name to warn about where money is headed. Some worry that if central banks tone down their interventions, financial markets could become tumultuous. The Fed is amid what it hopes is a long, gradual process of normalizing interest rates and unwinding its $4.5 trillion balance sheet of bonds it mostly accrued during its post-crisis stimulus programs.

“You have artificial prices supported by central banks, and investors are just following central bank money to buy assets and ride the wave,” Santschi said. “Why pay for management and research if you can just index and get the returns?”

So far in 2017, investors have been both following the stock market higher and continuing to pile into bond ETFs as well.

Equity funds have landed $190.6 billion in new money, while fixed income has garnered $82.9 billion, according to State Street. Despite a seemingly strong desire for global diversification, U.S.-focused funds have still raked in a healthy $86.7 billion, while international ETFs have seen nearly $100 billion in inflows.

“When no one is worried, they tend to easily throw caution into the wind, ignoring risks in the market and chasing winners absent of any focus on price paid,” Matthew Bartolini, head of SPDR Americas research at State Street, said in a report. “So while investors should enjoy the calm for as long as it lasts, they should also remember this is not a risk-less market.”

WATCH: Paul Singer explains why he thinks ETFs are “devouring capitalism.”