TheStreet's Jim Cramer, co-manager of the Action Alerts PLUS portfolio, posed an interesting question on CNBC's "Mad Dash" segment: "Why is the market so strong? What is the bid underneath?"

That bid, he said, is Japan.

"It's just crazy," Cramer said, citing a Real Money research report by James Gentile.

The Government Investment Pension Fund of Japan has taken in some cases enormous stakes in individual U.S. companies. The list includes $2 billion worth of General Electric (GE) - Get Report  stock, $1.5 billion in Alphabet (GOOGL) - Get Report , $100 million in Whirlpool (WHR) - Get Report , $180 million in Eaton (ETN) - Get Report and $150 million in Roper Technologies (ROP) - Get Report .

"Now, if you do these aggressively, you actually move these stocks," Cramer said, especially when the market has bouts of low trading volume. It seemed odd that some of these stocks have stayed afloat despite reporting so-so earnings, he added.

"This is concentrated buying by the government," he explained. Unlike the Federal Reserve, which opted to buy Treasury bonds, Japan has bought stocks -- individually and broadly -- as well as bonds and real estate.

Cramer questioned this tactic and said he was "astounded" to learn of it. The fund had to have been one of the biggest, if not the biggest, buyer of GE stock last month, he said.

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At the time of publication, Cramer's Action Alerts PLUS had a position in GOOGL and GE.