If you feel strongly about Vanguard S&P 500 ETF (VOO), buy it. The top 500 companies in the U.S. are far larger than companies in the rest of the market - as terran said, they make up 80% of the stock market. You could instead buy either VTI (Vanguard) or ITOT (iShares) and capture the entire stock market for a low fee (~3700 stocks, 0.03% expense ratio).
The key thing is what makes you start investing. I would prefer VTI, but if buying VOO makes you start investing, that's the better choice for you.