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Dark Matter: The Hidden Skeleton of the Cosmos


What holds galaxies together, controls their spin, and outweighs all the visible stars? Welcome to the mysterious realm of dark matter — the invisible glue of the universe. If you’re joining us now, catch up with previous posts: Spiral vs Elliptical Galaxies | Galaxies | What is the Universe?

Dark matter illustration

🔍 What is Dark Matter?

Dark matter does not emit, absorb, or reflect light, making it completely invisible. Yet, scientists know it exists because of its gravitational effects on visible matter.

🧠 Scientific Estimate: About 27% of the universe is dark matter. Only 5% is normal matter.

📈 Evidence for Dark Matter

  • Galaxy Rotation Curves: Stars in galaxies orbit faster than visible mass allows.
  • Gravitational Lensing: Light bends around unseen mass, revealing dark matter’s presence.
  • Cosmic Microwave Background: Tiny fluctuations suggest invisible matter affects early universe structure.

🧪 The Galaxy Rotation Curve Equation

According to Newtonian gravity, orbital velocity \(v\) at distance \(r\) from the galaxy center should be:

$$v = \sqrt{\frac{G M(r)}{r}}$$

But observations show that \(v\) remains nearly constant even at large \(r\), implying more mass (i.e., dark matter) distributed beyond the visible edge.

🔬 What Could It Be?

Several theories try to explain the nature of dark matter:

  • WIMPs: Weakly Interacting Massive Particles
  • Axions: Hypothetical ultra-light particles
  • MACHOs: Massive Astrophysical Compact Halo Objects like black holes or neutron stars

🧬 Mathematical Density Profile: The NFW Equation

The Navarro-Frenk-White (NFW) profile is a model of dark matter density in galactic halos:

$$\rho(r) = \frac{\rho_0}{\left( \frac{r}{r_s} \right)\left(1 + \frac{r}{r_s} \right)^2}$$

Where:

  • \(\rho(r)\) = density at distance \(r\)
  • \(\rho_0\) = characteristic density
  • \(r_s\) = scale radius of the halo

🪐 Why It Matters

Without dark matter, galaxies wouldn’t hold together. Structures in the universe wouldn’t form as they did. It's the hidden skeleton shaping the cosmos.

💡 Think about this: You, the stars, and everything you can see make up just 5% of the universe. The rest is hidden — in dark matter and dark energy.

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