The chart above shows the combined (bloated) balance sheets of the major central banks after years of quantitative easing. Eleven trillion dollars later, you can see clearly that central bank balance sheets have peaked, and will soon begin to decline. The Fed is already actively engaged in running off its bloated balance sheet. This began in October, and is set to accelerate in 2018. Other central banks will soon follow the Fed's lead. Global central banks are shifting from being the buyers of last resort for all sorts of toxic debt and equities to being sellers of those assets.
These developments aren't having an immediate negative impact on U.S. markets, which remain in rally mode into the holidays. But it's worth keeping a watchful eye on this as 2018 fast approaches. If global credit conditions keep tightening, it will surely result in much more volatility for markets.