When it comes to growth investing, market opportunity is a particularly important metric. The size of the market in which a business competes -- both now and in the future -- ultimately will determine how large that business can hope to become. And investing in a company that's early in its growth curve can lead to years of handsome profits as it fulfills its growth potential.

But whether or not a company reaches that potential is often decided by the strength of its competitive advantages. That's why investing in a company with a wide -- and, even better, expanding -- moat around its business can help you increase your odds of achieving the type of explosive gains that successful growth investing can deliver.

The challenge, of course, is finding those rare businesses that possess both a massive untapped market opportunity and dominant competitive position. Fortunately, we can invest in one such business today. Read on to learn more about it.

Image source: Zillow.

Zillow Group (NASDAQ:Z) (NASDAQ:ZG) is the largest online real estate marketplace. The company's collection of brands includes its namesake Zillow.com site as well as popular online real estate properties StreetEasy, HotPads, and Naked Apartments. Together, these sites allow house and apartment hunters to search more than 100 million for-sale and rental listings, compare Zillow's proprietary Zestimate home values, and connect with local real estate professionals.

Zillow solidified its position as the No. 1 player in online real estate by acquiring No. 2 rival Trulia in February 2015. The combined company's reach is unmatched, with Zillow Group commanding 63% of the online and mobile real estate audience, and an even more impressive 72% of the mobile-only category, according to comScore data. More than 166 million unique users visited Zillow's mobile apps and websites in March, a 22% year-over-year increase. And perhaps most telling of all, more Americans now search "Zillow" than "real estate," according to Google Trends. 

As they say in the advertising world, dollars follow eyeballs, and real estate professionals are willing to pay hefty sums to reach Zillow's ballooning audience. Sales in Zillow's premier-agent business jumped 25% year over year to $134.5 million on a pro forma basis, which assumes that Zillow owned Trulia during the entire reporting period. Average revenue per advertiser surged 40% to $487, and the number of premier agents who spend more than $5,000 per month on Zillow's ad platform soared 74%. 

Yet even after this torrid growth, Zillow Group accounts for only a small portion of the more than $12 billion that real estate agents spend on advertising every year. In all, management expects the company's full-year revenue to rise to a range of $825 million to $835 million, representing a 29% year-over-year increase at the midpoint. Even at the top end of the range, those figures represent less than 7% of annual agent ad spend.

With about 40% of homebuyers now turning to the internet to find their new property, according to the National Association of Realtors -- up from just 11% in 2003 -- that spending gap should narrow significantly in the years ahead, as advertising spend on Zillow's platform climbs to a level more commensurate with its dominant position as the leading online real estate marketplace.

Image source: Zillow.

Zillow's powerful network effects should further fuel its growth. Each additional user increases the value of Zillow's platform, as more potential customers attract more agents, and more agents list more homes for buyers and renters. This virtuous cycle should continue to expand Zillow's already unmatched reach and scale and position the company to grow exponentially as ad spending increasingly shifts to the web and mobile devices in the coming decade.

All told, with its dominant competitive position and massive growth potential, investors who buy Zillow's shares today are likely to be well rewarded.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.