Glossary of Astronomical Terms/dark matter

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The dark matter mystery started when Vera Rubin measured the light intensity of stars that we receive from distant galaxies and used this to compute the mass of the galaxy (abiet 90% of its expected mass, see mass-luminosity law). Then she compared that to the mass of the galaxy found from measurements made of those star's velocities using the Doppler shift formula in the orbital velocity-central mass formula (from Newton's Law of Gravitation). This comparison showed that the stars orbit much faster than their parent galaxy's gravity (based on its luminosity) can allow, which gave rise to the current mystery.

So apparently, galaxies have more matter in them than their visible light would indicate. What is amazing is that in order for galaxies to hold on to those stars moving at those speeds, they would need almost 10 times more matter than we can account for. This seems to hold for any galaxy we analyze. Hence, we don't know what 90% of the universe is made of. As of this writing we know of two things:

1. Dark matter's gravitational effects - from this we get a picture that most of the dark matter is found in a halo outside the galaxy. 2. It doesn't reflect light, heat, radio, X-rays, ... and we really don't know of anything that does that.

We've searched for MACHOS (MAssive Celestial Objects) such as black holes, neutron stars, brown dwarf stars, but found way to few to account for it all. We're thinking about WIMPs (Weakly Interacting Massive Particles), but don't know how to detect them. Neutrinos could fit this bill, as they've recently found to have some mass, but it doesn't look like its enough.

Finally this dark matter may be linked to dark energy, another mystery that seems to be causing the universe to expand at an accelerating rate.

In a way, this dark matter not only serves to declare our state of ignorance of the universe, but also challenges us to tackle this deep mystery. Let the exploration and discoveries begin!