The nature of dark energy and dark matter are two of the most outstanding questions in modern physics . I will begin this talk by presenting the DESI (Dark Energy Spectroscopic Instrument, https://www.desi.lbl.gov) survey which is currently collecting data and will provide the strongest constraints on the nature of dark energy in the next 5 years. I will present results from my group based on measurements of gravitational lensing that suggest intriguing differences between the low and the high redshift universe that could have profound implications for our understanding of dark energy and the Lambda Cold Dark Matter (LCDM) model. I will also present new results suggesting that the light from central galaxies is a much better tracer of halo mass than previously recognized and I will discuss how this effect might be used to improve optical cluster finding algorithms and to further tighten constraints on dark energy. In the second half of this talk, I will present the Merian survey (https://merian.sites.ucsc.edu), a 60 night program on the Blanco telescope in Chile to probe the nature of dark matter in dwarf galaxies.
