Astronomers using the Hubble Space telescope have viewed the Smith Cloud, a high velocity could of hydrogen gas. The Smith Cloud is a massive cloud of gas that is outside of the Milky Way galaxy and is traveling at 700,000 mph. The most unique aspect of the Smith Cloud is that its trajectory is well known.
Scientists believe that the Smith Cloud launched from the outer regions of the galactic disk about 70 million years ago and the cloud was first discovered in the 1960s. The person who discovered the cloud was Gail Smith who detected the radio waves emitted by the hydrogen.