Catalyzes the terminal and only committed step in triacylglycerol synthesis by using diacylglycerol and fatty acyl CoA as substrates. Required for storage lipid synthesis. a 1,2-diacyl-sn-glycerol + an acyl-CoA = a triacyl-sn-glycerol + CoA Optimum pH is 6.0-8.0. Glycerolipid metabolism; triacylglycerol biosynthesis. Belongs to the diacylglycerol acyltransferase family.