Eight years ago, recently elected U.S. President Barack Obama had inherited a mess: The U.S. economy was bleeding jobs, the American auto industry was on the brink of collapse, giant investment bank Lehman Brothers had folded and the stock market was in freefall.

On March 9, 2009, less than two months into Obama's presidency, the blue-chip Dow Jones Industrial Average dropped to 6,547, a level not seen since April 1997. The S&P 500 followed suit, plummeting below 700 to a 13-year low.

"At the worst of the financial crisis, the U.S. equity market was discounting a prolonged and severe U.S. recession and a prolonged period of falling earnings for U.S. companies," John Canally, chief economic strategist for LPL Financial, said in an interview. The panic on Wall Street was palpable.

And then it began to lift. For the Dow Jones, that Monday in March proved to be a nadir, the point on which Wall Street's second-longest bull market on record would ultimately rest.

Since then, the Dow has more than tripled in value. The S&P 500 has followed a similar trajectory, notching gains of nearly 250% and recording another all-time high in just the past week. The only longer bull market was the one that began in 1987 and continued through the collapse of the dotcom bubble in 2000.

Despite its dizzying highs, however, the expansion hasn't benefited all portions of the U.S. economy, and investors have second-guessed its longevity at every twist and turn.

"The current bull market has been the least loved, the most questioned and the most doubted bull that we certainly can recall in nearly 34 years of experience in the markets," said John Stoltzfus, chief investment strategist at Oppenheimer.

Part off the reason is that it has been such a slow-moving beast. Howard Silverblatt, senior index analyst at S&P Dow Jones Indices, said he calculates an annualized return of 17.02% during the expansion, compared with an average of 22.2% since the 1930s.

Not only were investors traumatized by the financial crisis, which had frozen global credit markets and required billions of dollars in government bailouts, they were forced to grapple with a steady stream of curveballs: a sovereign debt crisis in Europe, Britain's decision to leave the European Union and plunging crude oil prices.

Exploding investment in burgeoning sectors such as social media -- think Facebook(FB) - Get Report and Twitter(TWTR) - Get Report  -- even called to memory the overblown valuations of the dotcom bust.

Indeed, a record-breaking rally since Donald Trump's surprise victory in last year's U.S. presidential campaign overshadows the fact that just a year ago, worries were escalating over an impending bear market.

The stock market's resilience is largely thanks to the actions of a dynamic and responsive central bank. The Federal Reserve slashed interest rates to nearly zero in 2008, then kept them there for seven years and made billions of dollars in purchases of agency mortgage-backed securities.

The Fed "stepped in and prevented the Great Recession from becoming another Great Depression," Chris Zaccarelli, chief investment officer for Cornerstone Financial Partners, said in an email. "By keeping the Fed Funds rate close to zero and by purchasing bonds in the open market, they were able to keep interest rates extremely low for a long period of time, which allowed U.S. equities to embark on a long bull market."

Now, however, the question is whether there's enough gas in the tank to extend the bull market by another year. 

"Going forward, as the Fed begins to increase interest rates, the bull market will eventually run out of steam and succumb to the next bear market, but until we have another recession, it's unlikely to happen in the short run," said Zaccarelli.

Sam Stovall, chief investment strategist at CFRA, says the S&P 500 will continue to rise, though the angle of its ascent will dip. The firm targets 2,460 on the S&P 500 in the coming 12 months, nearly 4% above current levels.