Our understanding of the universe is changing. New observation techniques have become operational and the Hubble space telescope, finally cured of its bad eyesight, has revealed unsuspected things. In immense molecular clouds we have discovered star nurseries whose children still attached to their uterus by "umbilical cords". Infra-red telescopes are showing us the universe from an unprecedented angle. We will be able to see things as a whole. We are sailing towards a new world.
Our conception of the cosmos depends on observations. Without them, we would turn in sterile circles playing with equations. Observation bought about the cosmological revolution at the beginning of this century and will provoke the revolution of the now so close twenty-first century.
To account for these new observations, we must improve, and perhaps profoundly modify, our conception of the universe. We have always considered the universe to be made of clumps, hierarchically arranged. Galaxies are clumps of stars, and clusters of galaxies form clumps too. We expected to find bigger clumps, already named "superclusters" but instead we have discovered a strangely hollowed-out universe, structured on a very large scale like Swiss cheese (or rather, to be true to our friends the Swiss, like Emmenthal, since real Swiss cheese, Gruyere, does not have holes).
Galaxies, beginning with own Milky Way, lack the mass to balance centrifugal forces. On the basis of stars brilliant enough to make an impression on the plates of our optical telescopes, these "island universes" should have blown apart long ago and been scattered to the four winds of the cosmos. Something remains to discover which still escapes us -- perhaps stars of very small mass and luminosity, or unknown objects such as new particles. Perhaps what the superstring theoreticians call a "shadow universe", as suggested by John Schwarz of Caltech, Michael Green of Queen Mary College, London, or Nobel Prize winner Abdus Salam (in his contribution on the unification of electromagnetism and the "weak force"). A "shadow universe", they say, is not observable optically but revealing its presence through gravitational effects alone.