The invisible and mysterious form of matter that makes up most of the mass in the Local Group and significantly affects the motion and structure of its galaxies. Although dark matter does not emit, absorb, or reflect light, its presence is detected through gravitational effects.
Key points:
- Dark matter provides the gravitational glue that holds the Local Group together.
- The combined mass of all visible stars and gas in the Local Group is not enough to explain the motions of its galaxies—dark matter accounts for over 80–90% of the total mass.
- Each major galaxy, like the Milky Way and Andromeda, is surrounded by a massive dark matter halo that extends far beyond their visible edges.
- The orbits of satellite galaxies and the movement of galaxy pairs can only be explained if large amounts of dark matter are included in models.
- Dark matter also influences future events in the Local Group, such as the predicted collision between the Milky Way and Andromeda, by shaping their gravitational interaction.
Dark matter is essential to understanding the formation, stability, and evolution of the Local Group and the universe as a whole.