Transfers sialic acid from the donor of substrate CMP-sialic acid to galactose containing acceptor substrates. a beta-D-galactoside + CMP-N-acetyl-beta-neuraminate = an N-acetyl-alpha-neuraminyl-(2->6)-beta-D-galactosyl derivative + CMP + H(+) Belongs to the glycosyltransferase 29 family.