The nature and properties of dark matter and dark energy in the universe are among the outstanding open issues of modern cosmology. Despite extensive theoretical and empirical efforts, the question "what is dark matter made of?" has not been answered satisfactorily. Candidates proposed to identify particle dark matter span over ninety orders of magnitude in mass, from ultra-light bosons, to massive black holes. Dark energy is a greater enigma. It is believed to be some kind of negative vacuum energy, responsible for driving galaxies apart in accelerated motion. In this article we take a relativistic approach in theorizing about dark matter and dark energy. Our approach is based on our recently proposed Information Relativity theory. Rather than theorizing about the identities of particle dark matter candidates, we investigate the relativistic effects on large scale celestial structures at their recession from an observer on Earth. We analyze a simplified model of the universe, in which large scale celestial bodies, like galaxies and galaxy clusters, are non-charged compact bodies that recede rectilinearly along the line-of-sight of an observer on Earth. We neglect contributions to dark matter caused by the rotation of celestial structures (e.g., the rotation of galaxies) and of their constituents (e.g., rotations of stars inside galaxies). We define the mass of dark matter as the complimentary portion of the derived relativistic mass, such that at any given recession velocity the sum of the two is equal to the Newtonian mass. The emerging picture from our analysis could be summarized as follows: 1) At any given redshift, the dark matter of a receding body exists in duality to its observable matter. 2) The dynamical interaction between the dark and the observed matter is determined by the body's recession velocity (or redshift). 3) The observable matter mass density decreases with its recession velocity, with matter transforming to dark matter. 4