What’s the interplay between dark matter and baryons? Luminous and dark matter in massive galaxies. Implications for massive galaxy formation. Implications for the stellar IMF. Implications for Ho. Strong gravitational lenses provide unique insights into the structure of galaxies (and cosmology). Current samples show that: The halo of massive galaxies are well described by unperturbed NFW, ruling out compression (or expansion); State of the art cosmological simulations reproduce overall mass profile, but leave too much dark matter in the centers of galaxies; The stellar IMF of massive elliptical galaxies is inconsistent with popular models like Chabrier (non-universality). Roman Space Telescope will improve samples by 2 orders of magnitudes. Dramatic improvements in understanding galaxies and cosmology, with proper follow-up.