Data: Institute for Local Self-Reliance; Map: Naema Ahmed/AxiosThe cities and towns where Walmart gobbles up more than half of all grocery sales are concentrated in the South and the middle of the country, illustrating a broader division in U.S. retail.Why it matters: Retail has become one of the forces driving American inequality. While some cities build glitzy, revamped main streets and get same-day shipping, others have few options beyond Walmart super-centers and dollar stores.By the numbers:Walmart owns 50% or more of grocery sales in 203 U.S. markets, per an analysis by the Institute for Local Self-Reliance, a nonprofit research and advocacy group that opposes concentrated economic power.In 38 of those markets, Walmart has 70% or more of the grocery sales.And in some places, the retail giant has virtually wiped out all other competition. Walmart owns 95% of the market in Portales, New Mexico; 95% in Atchison, Kansas; and 90% in Guymon, Oklahoma.The bottom line: No grocer in U.S. history has ever been so dominant.Yes, but: As politicians and consumers alike raise concerns about the outsized economic power of big companies, Walmart keeps growing — largely under the radar."The political pressure just isn't there to investigate Walmart," says Sally Hubbard, a director at the Open Markets Institute and a former assistant attorney general in New York's antitrust bureau. "The places where Walmart has large grocery market share are not places where policy activists live."Go deeper: Two Americas, divided by how we shop