Dark matter and dark energy play crucial but distinct roles in shaping the evolution of the universe:
1. Dark Matter: Structure Formation and Gravity
Role in Cosmic Structure: Dark matter, which makes up about 27% of the universe, doesn't emit light but exerts gravitational attraction. Early in the universe, its clumps served as gravitational wells, pulling in normal matter and helping form galaxies and large-scale structures.
Galactic Stability: It holds galaxies together. Observations show stars in galaxies move faster than visible matter alone can explain—dark matter’s gravity accounts for this.
Cosmic Web: Dark matter is the scaffold upon which galaxy clusters and filaments formed. The universe's large-scale structure traces the distribution of dark matter.
2. Dark Energy: Accelerated Expansion
Dominant Force Today: Dark energy comprises about 68% of the universe and is driving the universe's accelerated expansion.
Cosmic Acceleration: Around 5 billion years ago, the repulsive force of dark energy overcame the gravitational pull of matter, leading to accelerated expansion. This was discovered through observations of distant supernovae.
Future Evolution: If dark energy remains constant or increases, the universe could continue expanding forever, potentially ending in a "Big Freeze" where galaxies move so far apart that stars burn out and the universe becomes dark and cold.
Summary:
Dark Matter helps structure form through gravitational attraction.
Dark Energy drives the expansion of the universe, affecting its ultimate fate.
Their combined effects govern the universe’s past, present, and future dynamics.