The CMB presents a unique probe of dark matter physics. Ultra-light axions of mass around 10^{-22} eV are a promising dark matter candidate well motivated by high energy physics. While detection experiments hold great promise for axion detection, small-scale measurements from the cosmic microwave background (CMB) provide a window into the dark sector that is not probed by detector experiments. Experiments like the Planck satellite allow us to constrain the axion contribution to the total dark content energy density over a wide range of masses. Future experiments will go even further. The Simons Observatory (SO) is a new experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes (SATs) and one large-aperture 6-m telescope (LAT), with a total of 60,000 cryogenic bolometers. I will highlight some of the current and proposed constraints on axions from the CMB and large scale structure, and present forecasts from upcoming CMB probes on ultralight axions.