Catalysis of the reaction: 1,2-diacyl-sn-glycerol + UDP-D-galactose = 1,2-diacyl-3-beta-D-galactosyl-sn-glycerol + H+ + UDP.
UDP-galactose:1,2-diacyl-sn-glycerol 3-beta-D-galactosyltransferase activity