One parameter which allows us to decide upon the possible geometric shape of the Universe and establish the ultimate destiny of the Universe is the average density of the matter present in the Universe.

Imagine that we introduce the parameter Omega , equal to the ratio between the density of the Universe and a specific value called critical density. If Omega  > 1, space has a spherical geometry and the Universe will expand and then close in on itself. This is called a closed Universe. If Omega = 1, space is euclidean, its geometry is flat and the Universe will expand forever. If Omega < 1  the Universe is said to be open, it will expand forever and its geometry is hyperbolic. It is therefore extremely important to evaluate the total mass of the Universe and distinguish between the various forms of the Cosmos and its possible future.

Inflationary cosmological models predict a value of 1 for the Omega parameter, and a flat Universe. Given that, scientists deduce, from the visible quantity of matter, an Omega value of 0.005, we can say that dark matter should make up 99.5% of the mass of the Universe. But is it possible to verify that the density of matter really corresponds to the critical density, as foreseen in the theory of inflation?

The total amount of matter required in order to keep stars in galaxies and galaxies in superclusters is estimated to be 35% of that needed to obtain a Universe of euclidean geometry. It is also believed that this matter is subdivided as follows:

On the other hand, other studies (most recently the WMAP experiment) have demonstrated that the small fluctuations in temperature in the microwave background radiation have the characteristics expected in the case of a Universe with an energy density equal to the critical density.
What a strange Universe we live in! The idea that the visible Universe forms only half a percent of everything that exists is hard to swallow.

So what is the remaining 65% of "density" which has nothing to do with either visible matter or dark matter? We will look at this in the next section.
Carl Sagan was right when he said that we are made of stars, but now astrophysicists are telling us that the Universe is not!

Fig. 2:This figure shows the fluctuations in the temperature of the background cosmic radiation revealed by the WMAP (Wilkinson Microwave Anisotropy Probe)experiment. These fluctuations are very small: deviations of 1/1.000.000 from the average temperature value (~ 2.73 oK).In the image in false colours, the red zones indicate the hottest areas and the blue zones the coldest, in relation to the average value. It is a projection on an oval map of the distribution of electromagnetic radiation in the microwaveregion the entire sky, and it shows what the Universe was like 379.000 years after the Big Bang. The small fluctuations then gave rise to galaxies, clusters of galaxies, etc. (Credit:NASA/WMAP Science Team)