The bulls are on full parade Monday morning in U.S. equity markets, as the Dow Jones Industrial Average topped 16,000 for the first time in history. The HW 30 (HW30) equity index of firms driving the U.S. housing economy also broke higher Monday morning, jumping more than 4 points to take the index up to 1051.14 at the time of writing.

The advance of U.S. equity markets in the back half of this year has been breathtaking -- but it's also leading analysts and investors to wonder if, and when, the party will finally end. Per the WSJ:

Kent Engelke, chief economic strategist at Capitol Securities Management, with more than $4 billion in client assets, said there is a nervousness among investors about the market's unrelenting advance. "However, perhaps this nervousness is bullish, as market advances never end if there is a growing wall of worry."

That nervousness is leading some to ask if there's a bubble? And, well, if you have to ask:

There are so many superlatives in the stock market that it is easy to forget the S&P 500 has set 36 records this year on the way to its 26% gain—or that the federal debt-ceiling crisis threatened to disrupt the economy just a month ago.