DARK MATTER
the invisible that fills the universe
By fitting a theoretical model of the composition of the universe to the
combined set of cosmological observations, scientists have come up with
the composition that we described above, ~68% dark energy, ~27% dark
matter, ~5% normal matter.
What is dark matter?
We are much more certain what dark matter is not than we are what it is.
First, it is dark, meaning that it is not in the form of stars and
planets that we see. Observations show that there is far too little
visible matter in the universe to make up the 27% required by the
observations. Second, it is not in the form of dark clouds of normal
matter, matter made up of particles called baryons. We know this because
we would be able to detect baryonic clouds by their absorption of
radiation passing through them. Third, dark matter is not antimatter,
because we do not see the unique gamma rays that are produced when
antimatter annihilates with matter. Finally, we can rule out large
galaxy-sized black holes on the basis of how many gravitational lenses
we see. High concentrations of matter bend light passing near them from
objects further away, but we do not see enough lensing events to suggest
that such objects to make up the required 25% dark matter contribution.