In previous work, q-theory was introduced to describe the gravitating macroscopic behavior of a conserved microscopic variable q. In this article, the gluon condensate of quantum chromodynamics is considered in terms of q-theory. The remnant vacuum energy density (i.e., cosmological constant) of an expanding universe is estimated as K3QCD/E2Planck, with string tension KQCD≈(102MeV)2 and gravitational scale EPlanck≈1019GeV. The only input for this estimate is general relativity, quantum chromodynamics, and the Hubble expansion of the present Universe.