Dark matter in galaxies is a crucial component that helps explain how galaxies move and stay together. Although it does not emit, absorb, or reflect light, its gravitational effects reveal that it makes up most of a galaxy’s total mass.
Astronomers discovered the need for dark matter by studying galactic rotation curves — graphs showing how fast stars orbit at different distances from a galaxy’s center. According to visible matter alone, stars farther from the center should move more slowly. However, observations show that the outer stars move just as fast as inner ones, meaning there must be an unseen source of gravity holding the galaxy together.
This invisible mass is what we call dark matter, forming a large, roughly spherical halo around each galaxy. It prevents galaxies from flying apart despite their rapid rotation.
Key points:
- Dark matter halos extend far beyond the visible edges of galaxies.
- It makes up about 85% of a galaxy’s total mass, while stars, gas, and dust make up only a small fraction.
- Without dark matter, galaxies couldn’t exist in their current form, as their visible matter alone wouldn’t generate enough gravity to keep them stable.
In short, dark matter is the hidden framework of galaxies, providing the gravity needed to explain their flat rotation curves and ensuring the stability and structure of galaxies throughout the universe.