The observed clustering of about a million galaxies in the Universe. Fig. 3.7

Atoms and molecules, and even neutrino-like particles, are far from all there is pervading outer space. The Radiation exists in all wave lengths. The most pervasive and most significant contributor to the total energy density of the Universe is the sea of microwave photons left over from the hot early stages of the Universe. As the universe has expanded, these photons have lost energy, increased in wavelength and cooled to a temperature only 2.7 degrees above absolute zero. There are about 411 of these phjotons in every cubic centimeter of space. That is there are roughly one billion of these photons for every atom in the Universe.

Our detailed probing of the distribution of matter and radiation in the Universe shows that, as we survey larger and larger volumes of the Universe, the density of material that we find keeps falling until we get out beyond the dimensions of clusters of galaxies (Fig. 3.7). when we reach the scale the clustering of matter starts to fade away and looks more and more like a tiny random perturbation on a smooth sea of matter with a destiny of about one atom in every cubic meter. As we look out to the largest visible dimensions of the Universe we find that the deviations from perfect smoothness of the matter and radiation remain at a low level of just one part in one hundred thousand. This shows us that the Universe is not what has been known as fractal, with the clustering of matter on every scale looking like a magnified image of that on the next large scale. The clustering of matter appears to peter out before we reach the limit of our telescopes. This is a reflection of the fact that these large aggregates of matter take time to assemble under the influence of gravitational attractions. There is only a finite time available for this process and so its extent is limited.

The Universe appears to be a system of very low density wherever we look. This is no accident. The expansion of the Universe weds its size and age to the gravitational pull of the material that it contains. In order that a universe expands for long enough to allow the building blocks of life to form in the interiors of stars, by a sequence of nuclear reactions, it must be billions of years old. This means that it muse be billions of light years in extent and possess a very small average density of matter and a very low temperature. The low temperature and energy of its material ensures that the sky is dark at night. Turn off our local Sun and there is just too little light around in the Universe to brighten the sky. The night is dark, interspersed only by pinpricks of starlight. Univferses that contain life must be big and old, dark and cold. If our Universe was less of a vacuum it could not be an abode for living complexity. ~ Pages 113 & 114 (The Book of Nothing)