SpaceX is expected to begin trading tomorrow, and the excitement surrounding the IPO is reaching levels we rarely see. Demand has reportedly surged far beyond the initial offering size, retail investors are scrambling for allocations, and financial media can't seem to talk about anything else. Some reports even suggest investors have been selling other technology holdings to free up cash for the deal. Whether those stories are overblown or not, they tell us something important: Wall Street has been waiting for the next "must-own" event.

At a certain size, a company stops being just a stock. It becomes an index story.

Most investors are focused on the IPO itself—what price it opens, whether it pops on day one, and whether the valuation is justified. But those questions matter most to traders. Long-term investors should probably be paying closer attention to what happens next. Because eventually, a company this large begins to impact portfolios far beyond the people who intentionally buy shares.

At some point, SpaceX will likely find its way into major benchmarks. And when that happens, millions of investors will become shareholders without ever placing a trade. Their retirement accounts, index funds, and ETFs will do the buying for them. That's one of the biggest shifts in modern markets. The largest flows no longer come from investors researching companies one by one. They come from passive money automatically following the indexes.

That's where the real portfolio lesson lives. Once a company reaches a certain size, valuation isn't the only force driving demand. Inclusion matters. Index funds aren't asking whether SpaceX is cheap, expensive, or fairly valued. They're asking whether they need to own it. And when trillions of dollars are benchmarked to the same indexes, that creates a powerful tailwind that didn't exist a generation ago.

For investors, this is a good reminder to understand what's actually inside your portfolio. Many people believe they're broadly diversified across hundreds of companies. In reality, an increasing share of market returns is being driven by a surprisingly small group of giants. The companies that dominate the indexes increasingly dominate investor outcomes as well.

Whether SpaceX ultimately grows into its valuation remains to be seen. But the bigger story isn't the rocket launches, the Mars headlines, or even the IPO itself. It's the continued concentration of capital into a handful of companies that become large enough to pull entire indexes—and the investors tracking them—along for the ride.

The IPO gets the headlines. Index inclusion is where the real money moves.