The number of data centers worldwide peaked at 8.55 million in 2015, according to IDC. That figure began declining last year, and is expected to drop to an expected 8.4 million this year. By 2021, the research firm expects there to be 7.2 million data centers globally, more than 15% fewer than in 2015.

Data centers decline

The global square footage of data centers, after recent boom times, is also expected to slide. In 2013, data centers totalled 1.6 billion square feet. That was when big service providers like Amazon, Microsoft and Google were building huge data center complexes -- pushing square footage globally to 1.8 billion this year.

But IDC expects that number to decline from now on. Cloud adoption is a major reason for the trend.

Consider the adoption of Office 365, said Tad Davies, who heads consulting services at Bick Group, a data center consultancy. "Easy to move to and eliminates infrastructure in my data center," he said. "Same for CRM."

Consolidation is also playing a role, said Davies, as are new approaches to computing. New firms are adopting "cloud first" strategies, he said. "As they grow into larger organizations, the data center is never created."

Large users -- especially the U.S. government -- have been shrinking their data center space to drive efficiency. Better server utilization often means more consolidation.

While the biggest decline is affecting in-house data centers, said IDC, service provider data centers continue to expand. But even there, the pace of growth is moderating as the market matures.

Despite stagnant growth, data centers are still needed, Davies said. That's because there's limits to what can go into the cloud.