What is the name of the phenomenon of cool gas clouds falling onto the Milky Way at high speeds?

The phenomenon of cool gas clouds falling onto the Milky Way at high speeds is known as high-velocity clouds.

High-velocity clouds, or HVCs, are large accumulations of gas moving unusually fast relative to the surrounding Milky Way material. Their velocities are typically more than about 70–90 kilometers per second relative to the local standard of rest. Many are found in the Galactic halo, well above or below the plane of the disk.

The clouds were first noticed as dense pockets of gas outside the Galactic plane in the mid-1950s and were later detected through the radio emission of neutral hydrogen. Some HVCs contain relatively cool hydrogen, while the surrounding halo is much hotter and more diffuse. As clouds lose energy, they can condense and fall toward the Galactic disk.

These clouds matter because they supply fresh gas for future star formation. Their origins are varied: some may be infalling material from outside the Galaxy, while others may be gas expelled by stellar feedback and returning in a galactic fountain. They are not the Oort Cloud, molecular clouds, or the Magellanic Clouds.