How much Dark Matter

Cosmologists like to talk about the amount of matter in the universe. This is expressed in a term known as Omega. It is quite simple to understand. Omega is defined so that a closed universe or a universe that is massive enough to eventually collapse back onto itself is said to have an Omega larger that 1. And an open universe or one that expands outward forever and will never collapse onto itself is said to have an Omega of less than 1. Something in between these two known as a ‘flat’ universe, one that perfectly balances between the two has an Omega=1.

The amount of visible matter in the universe is about Omega=0.05. Not very much there at all. Physicist like to believe that for the universe, the total of all of the mass is Omega=1. This would mean that dark matter makes up the other Omega=0.95. Or, 95% of the universe would be dark matter! More realistically, there isn’t too much evidence for Omega being larger than about 0.4. This would make the amount of dark matter be Omega=0.35. And that means that 88% of our universe is complete mystery.