With the three major indexes slipping over the past few weeks, now may not seem like the best time to invest in stocks. It's always more tempting to buy when we see a particular stock or asset rising, as we can easily imagine our returns if the momentum keeps going. The idea is to hop on the bandwagon and immediately see your investment take off.

But as strange as this may sound initially, to truly score an investing win, one of the best things to do is consider buying during these periods of uncertainty. Why? Because quality stocks and other assets may be trading at bargain prices, meaning you can snap them up for a song and go on to benefit once they recover and advance over the long term.

It's important to remember that, generally, elements troubling the market -- from a government policy decision to rising inflation or a recession -- won't last forever. Today, investors are concerned about President Donald Trump's tariffs on imports from China, Canada, and Mexico and the impact that will have on the economy and corporate earnings.

This may, indeed, represent a headwind, but quality companies will be able to manage these times and come out ahead. Considering this, is the SPDR S&P 500 ETF Trust (SPY 0.56%) -- a bet on the S&P 500 -- the smartest investment you can make today? Let's find out.

Image source: Getty Images.

What's an ETF?

First, let's talk a little bit about this sort of investment. It's an exchange-traded fund (ETF), an instrument that includes many different stocks based on a particular theme, such as retail or biotech, or according to the player's presence in a particular index, the latter being the case with the fund we're talking about today.

Like stocks, ETFs trade daily on the market. So, if you're familiar with buying stocks, you can go about purchasing an ETF in exactly the same way. The one thing to be aware of is that ETFs do come with a management fee, expressed as an expense ratio. To preserve your winnings over time, go for an ETF with an expense ratio of less than 1%. With a ratio of 0.09%, the SPDR S&P 500 ETF largely fits the bill.

Now, we'll consider this ETF specifically and whether it's a smart buy for you. The SPDR S&P 500 ETF tracks the S&P 500's performance, so it truly is a bet on the overall stock market. It's important to remember that this index includes the top companies driving today's economy and makes adjustments regularly to ensure this is always the case. So, by investing here, you'll always be exposed to the leaders of the moment.

The S&P 500's recent performance

Of course, during market downturns, you'll likely see this ETF fall -- as is the case today. Moving in lockstep with the S&P 500, it's slid more than 6% over the past two and a half weeks. During times like these, certain individual stocks in your portfolio may be more likely to outperform. For example, as the overall market declined, stocks, including beverage giant Coca-Cola and pharma powerhouse AbbVie, advanced over the past month. This is why it's a great idea to diversify across companies and industries -- and across stocks and ETFs.

SPY data by YCharts.

But if you can make only one investment right now, the smartest move may be to pick up a few shares of the SPDR S&P 500 ETF, and here's why. The price, considering recent declines, has come down. But more importantly, this purchase offers you exposure to an index that has demonstrated resilience 100% of the time throughout its history.

After every period of decline, the index has gone on to recover and soar over time. In fact, the S&P 500 has delivered an annualized average return of more than 10% since its launch as a 500-company index in the late 1950s.

It's impossible to time the market and pick up a stock or an ETF at its very lowest point, but here's some good news: The S&P 500's track record offers us reason to be optimistic about returns over time if we invest in it at any point. And that's why the SPDR S&P 500 ETF Trust makes a fantastic buy right now -- even if the S&P 500 falls further in the short term.