NEW YORK/DALLAS—Walmart Inc. plans to expand its advertising business, adding space for ads at its U.S. stores and sharing its vast trove of shopper data with brands, the world’s largest retailer said on Thursday, looking to challenge rival Amazon.com Inc. as a media powerhouse. The efforts are key to Walmart’s aggressive plan to achieve a more than tenfold increase in its advertising business within five years, according to a source familiar with the matter. Reuters previously reported Walmart’s annual advertising revenue was expected to be nearly $1 billion in 2020. The Bentonville, Arkansas-based company, with nearly 5,000 stores nationwide, has set a goal to become one of the top 10 advertising platforms in the United States within the next five years, a “slightly conservative” goal, the source said. Walmart has expanded its advertising business after years of sputtering progress, even as a deal to buy a 7.5 percent stake in …