The Local Group is a gravitationally bound group of galaxies that includes the Milky Way, the Andromeda Galaxy and about 54 other smaller galaxies. The Local Group comprises more than 85% of all the mass in the observable Universe. Its gravitational attraction holds together a region with a diameter of about 10 million light-years. The three largest members of the group are M31 (the Andromeda Galaxy), our Milky Way and M33 (the Triangulum Galaxy). The combined mass of these three galaxies is about 1.1×1012 solar masses, or about twice that of our own Milky Way alone.
The Local Group is thought to have formed from density fluctuations in the very early Universe. As the Universe expanded and cooled, matter began to clump together under its own gravity to form clouds of gas and dust known as protogalaxies. These protogalaxies then coalesced to form larger galaxies like those we see today. Our own Milky Way was probably one of the first galaxies to form, with the Andromeda Galaxy being among the last.
The distances between individual galaxies within the Local Group vary from just a few hundred thousand light-years for some of the smaller dwarf elliptical galaxies, up to several million light-years for others such as M31 and our own Milky Way. Despite their name, there is no clear demarcation between “Local” and “non-local” groups – it is simply a convention based on where our galaxy happens to lie in relation to others. In fact, there are many groups and clusters beyond our own Local Group scattered across the Universe; some containing just a handful of galaxies while others comprise hundreds or even thousands.
While most groups and clusters are held together by their mutual gravitational attraction, there are also many examples where this appears not to be enough – particularly at larger scales where galaxy motions are dominated by dark energy rather than gravity. It is not yet clear whether or not our own Local Group will eventually succumb to this cosmic acceleration and be torn apart over time, or whether it will remain bound forevermore