We suggest a specification for dark matter. We suggest concepts that associate with gravitational repulsion between objects. Together, the dark-matter aspects and gravitational aspects seem to explain data that may otherwise be unexplained. Some of the data pertains to ratios of dark-matter effects to ordinary-matter effects regarding galaxy evolution, depletion of cosmic microwave background radiation, and the compositions of galaxy clusters. Some of the data pertains to eras in the evolution of the universe. We assume that nature includes six isomers of a set of most elementary particles. One left-handed isomer underlies ordinary-matter stuff. One similar right-handed isomer underlies some not-cold dark-matter stuff. The other four isomers underlie cold-dark-matter stuff. A novel use of multipole notions and special relativity points to aspects of gravity that repel objects from each other. We suggest that the dark-matter and gravitational aspects can provide bases for tuning cosmological models of the universe.