1. Flat Rotation Curves:
* Expected Behavior: According to Newtonian gravity, the orbital velocity of stars and gas in a galaxy should decrease with increasing distance from the galactic center. This is because the gravitational force decreases with distance, and thus the orbital velocity should fall off as the inverse square root of the distance.
* Observed Behavior: However, observations show that galactic rotation curves remain surprisingly flat even at large distances from the galactic center. This means that stars and gas are orbiting much faster than expected based on the visible matter alone.
2. Discrepancy between Luminosity and Mass:
* Expected Behavior: If we only consider the visible matter in a galaxy (stars, gas, and dust), the mass distribution should closely match the luminosity distribution. This is because the brighter regions of a galaxy should also be the most massive.
* Observed Behavior: However, the observed flat rotation curves indicate that galaxies must contain far more mass than can be accounted for by the visible matter alone. This discrepancy between luminosity and mass suggests the existence of a large amount of invisible matter – dark matter.
3. Extended Halo:
* Expected Behavior: If dark matter were concentrated in the central region of a galaxy, we would expect the rotation curve to start decreasing at some point.
* Observed Behavior: The fact that rotation curves remain flat to large distances suggests that the dark matter is distributed in a much larger, extended halo around the visible galaxy. This halo is thought to be much larger than the visible galaxy itself.
In summary: The flat rotation curves of galaxies, along with the discrepancy between their luminosity and mass, strongly suggest the presence of a significant amount of dark matter in a large, extended halo around the visible galaxy. This dark matter provides the extra gravitational pull needed to explain the observed high velocities of stars and gas in the outer regions of galaxies.
