The formation of ultra-diffuse galaxies (UDGs) is not well-understood. UDGs have large sizes (several kpc) for their dwarf-galaxy-like stellar mass, and have a wide range in dark matter content and other observed properties. In particular, NGC1052-DF2 and NGC1052-DF4 are two puzzling UDGs: they have low velocity dispersions indicating little to no dark matter content, as well as overly luminous and large globular clusters compared to the Milky Way population. Recently, we used the Keck Cosmic Web Imager (KCWI) to measure the velocity dispersion of the diffuse stellar light in NGC1052-DF4. High-resolution spectroscopy confirmed that the stellar kinematics are consistent with the globular cluster velocity dispersion, and rules out a normal NFW dark matter halo. We proposed that the two galaxies lacking dark matter formed jointly in the aftermath of a single bullet-cluster-like collision of dwarf progenitors. This is the first formation theory that can explain all the abnormal properties of both galaxies. I will also introduce the ongoing Dragonfly Ultrawide Survey. This survey covers the entire SDSS footprint in g and r band, and I will show examples of initial data. We will conduct a systematic search for large, nearby UDGs with the Ultrawide Survey.