Dark matter halos and their surrounding subhalo populations mirror the structure and evolution of galactic systems like our Milky Way and its nearby dwarf galaxies. Modeling how these structures change for different models of dark matter can better inform our understanding of cosmological theory. I investigated long-scale dark matter structure evolution using COZMIC, an N-body cosmological simulation suite, with Ethan Nadler and Vera Gluscevic through the Simons-NSBP Scholars Program at the Flatiron institute. I examined halo and subhalo mass distribution around Milky Way-like galaxies over long timescales, and produced first measurements of the time-dependence of the subhalo mass function for beyond-CDM models.