Why is knowing how much dark matter is out there important? Well the shape of the universe is dependent on the amount of matter present or what is called the Critical Density of the Universe, denoted by Omega. If Omega is less than one then the universe is thought to be an “open universe” and has negative curvature like a torus. In this scenario the universe will continue to expand forever because there isn’t enough mass present to slow down the outward expansion caused by the Big Bang.

If on the other hand Omega happens to be larger than one, then the universe is said to be a “closed universe” and has positive curvature like that of a sphere. In this scenario the universe will expand to a point then the gravitational pull of the universe will cause it to collapse in on its self into what is called the Big Crunch. But for the special case that the Omega equals one, the universe has no curvature and is said to be flat.

	Recent observations from probes such as WMAP, has determined that the universe is flat from which it follows that the mean energy density in the universe is equal to the critical density (within a 2% margin of error). This is equivalent to a mass density of 9.9 x 10 -30 g/cm 3, which is equivalent to only 5.9 protons per cubic meter.
Image of cosmic microwave background radiation taken by WMAP

 

Of this total density, we now know the breakdown to be:

> 4% Atoms, 23% Cold Dark Matter, 73% Dark Energy. Thus 96% of the energy density in the universe is in a form that has never been directly detected in the laboratory. The actual density of atoms is equivalent to roughly 1 proton per 4 cubic meters.

> Fast moving neutrinos do not play any major role in the evolution of structure in the universe. They would have prevented the early clumping of gas in the universe, delaying the emergence of the first stars, in conflict with the new WMAP data.

> The data places new constraints on the Dark Energy. It seems more like a "cosmological constant" than a negative-pressure energy field called "quintessence". But quintessence is not ruled out.

(Part of the previous text was copied from http://map.gsfc.nasa.gov/m_uni/uni_101matter.html, they said it better than I ever could)

 

So most of our universe is of a material we know little about, have never seen or experimented with in a laboratory. The universe is a much stranger place than we have ever imagined. Science has barley scratched the surface of existence and the deeper we explore the more we realize that we have no clue.