Involved in the synthesis of the major structural component of photosynthetic membranes. a 1,2-diacyl-sn-glycerol + UDP-alpha-D-galactose = a 1,2-diacyl-3-O-(beta-D-galactosyl)-sn-glycerol + H(+) + UDP By light and cytokinin. Belongs to the glycosyltransferase 28 family.