Dark Matter

Dark matter exists. We just don't know what it is.


You have probably heard about dark matter but aren't quite sure what is meant by it. Unlike a black hole, dark matter objects do not have a large gravity field preventing radiation from escaping. Dark matter, by definition, is matter in the universe that does not give off radiation.


Dark matter can reflect radiation, if radiation falls on it. However, in most cases, the reflected light is not bright enough to be seen from Earth. So, how then, do we know that dark matter exists?


The first evidence we ever had of dark matter is looking at the orbital velocity of stars in the plane of the Milky Way. Based on just observation alone, we expect that the mass of the galaxy tapers off as we go farther from the center of the galaxy. However, when measuring the orbital speeds of star at the edges of the galaxy, we see that the speeds do not match to what we expect based on the light we see alone.

So what is dark matter? There are a few ideas and some are more reasonable than others. I'll list them here and will go over in more detail over the next few posts about each of the theories.

• MACHOs - Massive Compact Halo Objects
• WIMPs - Weakly-Interacting Massive Particles
• Brown Dwarfs
• Planets
• Neutrinos