According to all current scientific theories, recently confirmed by the American Astronomical Society reunion in Texas in January 2012, the vast majority of the Universe is formed by dark energy, whose discovery surprised the scientific community a decade ago.
This mysterious type of energy, with a suggestive name, is present in the whole space and produces a pressure that generates a repulsive force by which the Universe’s expansion, after the Bing Bang, is not slowed down, which would seem logical, but it accelerates.
Composite image of the galaxy cluster CL0024+17, taken by the Hubble space telescope, that shows the creation of a gravitational lensing effect supposedly due, in large part, to the gravitational interaction with dark matter —Image NASA
Even though they share the adjective, it should not be confused with the dark matter. In astrophysics, this is how it is called the hypothetical matter that does not emit enough electromagnetic radiation to be detected with current technical means, but whose existence can be inferred from the effects it has on ordinary or visible matter, such as stars or galaxies.
The most precise calculations, made by the WMAP satellite, show that 72% of all that exists is dark energy, 23% is dark matter while only a 5% is ordinary matter, from what we currently know, formed by protons and neutrons.