The future of the
Universe depends on its density since this affects its rate of expansion. If this is greater than
the critical density the Universe is closed (the inevitable Big Crunch) while if it is less the
Universe will be open – expansion will go on forever at an increasing rate.
The
density depends on the mass of the Universe and when estimates of this were made based
on the luminous matter in the Universe alone it was found that the resulting value for density
was much too low. It was therefore suggested that there must be a lot of matter in the
Universe that we could not see and this has been named Dark Matter.
There is
thought to be a vast amount of dark matter in the Universe, up to 90% of its total mass! In our
galaxy it has been estimated that the dark matter has a mass of around 2x1011 solar
masses.
In the 1933 Zwicky and Smith both observed two relatively nearby clusters,
the Coma cluster and the Virgo cluster. They looked at the individual galaxies in the clusters
and measured and the velocities of these galaxies. They found was that the velocities of the
galaxies were about a factor of ten to one hundred larger than they expected.
Observations were first made by Rubin and W.K. Ford of the Andromeda galaxy.
They then measured the orbital velocities for hydrogen clouds in over 60 galaxies and found
the velocity does not decrease at large distances from the centre as would be expected if the
centripetal acceleration were simply due to the gravitational pull of luminous matter in the
galaxy alone.
As to exactly what dark matter is and whether it is 'hot' or 'cold' there
is as yet no firm conclusion.
Dark matter may be objects named MACHOs (Massive
Compact Halo Objects) such as Jupiter sized planets, brown dwarfs and even black holes.
Much of the hot dark matter may be neutrinos because even if their mass is very
small (less than a thousandth of that of the electron) there are so many of them that it would
account for all the gravitational effect of dark matter.
Cold dark matter could be
composed of WIMPs (Weakly Interacting Massive particles). These are particles with a mass
of over 100 times that of a proton.