The Milky Way’s disk spans at least 100,000 light-years and probably contains several hundred billion stars, which makes our home galaxy a giant.
However, the Milky Way is surrounded by a “halo” of cold dark matter, which produces no detectable energy but that reveals its presence by exerting a gravitational pull on the visible matter around it. The dark matter probably consists of some type of subatomic particle created in the Big Bang. This halo probably spans 250,000 light-years, and perhaps a great deal more.
The Milky Way is the second-largest galaxy (but perhaps the most massive, or “heaviest”) in what astronomers call the Local Group. The slightly larger Andromeda Galaxy and many smaller galaxies also populate the group, which spans about 6.5 million light-years.