There are some basic facts everyone should know about US paper currency: 

• Most of the value of US paper currency is $100 bills.

• The fraction of the value of US paper currency that is $100 bills is increasing.

• Most US paper currency—in particular $100 bills—is abroad.

Here is how Jo Craven McGinty lays it out in her July 6, 2018 Wall Street Journal article: 

Last year, according to figures published by the Fed, $1.6 trillion was in circulation, including $1.3 trillion in $100 bills, or 80% of the total. In 1997, $458 billion circulated, including $291 billion in $100s, or 64% of the total.

For at least two decades, the value of circulating currency has increased an average of 6% a year. Even when adjusted for inflation, the total value has more than doubled since 1997, and the total value of $100s is nearly triple what it was then. 

...  the bulk of U.S. cash sent abroad is in the form of $100 bills.

Up to two-thirds—or as much as $1.07 trillion—is held abroad. About $80 billion is held domestically by depository institutions. And the rest—as little as $453 billion—is in the hands of domestic businesses and individuals. 

For related posts, see the links in "How and Why to Eliminate the Zero Lower Bound: A Reader’s Guide."