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Correction: An earlier version of this story misstated the size of Stop & Shop's 'dark store' in Massachusetts.

(CNN Busines) —  

America’s top supermarkets are facing a new challenge: Grocery aisles in stores aren’t suited to meet the growing demand for online orders.

So Walmart, Albertsons, Stop & Shop, Meijer, Hy-Vee and others are building automated mini-warehouses inside their stores and opening up “dark stores” — locations that look like supermarkets but are closed to customers — to make deliveries and prepare pickup orders.

Although only around 5% of US shoppers buy their groceries online today, analysts expect that number to rise in the coming years.

Walmart has trained more than 30,000 workers to fulfill customers’ online grocery orders, but in some cases that has led to congested aisles with workers in its own stores during peak shopping times, such as weekends.