Why Do We Think Most of the Universe is Missing?
About 380,000 years after the big bang, the universe cooled enough to allow protons and electrons to combine to form neutral hydrogen atoms. This is called recombination. Photons that had previously bounced around between free protons and electrons suddenly had nowhere to go and were released, flooding the universe with cosmic background radiation.
As the universe expanded over the next 13.8 billion years, this radiation cooled further and now takes the form of microwaves and infra-red radiation. Its temperature is incredibly uniform across the sky, but it does show some very small differences, of just a few hundredths of a degree.
These variations are consistent with the most widely accepted model of the big bang. Detailed analysis using this model suggests that 68% of the mass-energy of the universe today consists of dark energy, the energy of ‘empty’ space. Dark energy is responsible for accelerating the expansion of the universe, as shown by analysis of very distant supernovae. About 27% is dark matter, an invisible form of matter detectable only through its gravitational effects. Nobody known what dark matter is, but there is some hope that experiments at CERN’s Large Hadron Collider may provide some clues.
About 5% is ordinary matter, or what we used to think of as ‘the universe’ not so very long ago. The universe is mostly missing.