THE DARK matter which composes up to 99 per cent of the mass of the
Universe may already have been ‘seen’ by astrophysicists. Its ‘signature’
may be the excess of positrons in the background of cosmic rays – high-energy
particles from space. According to some theories, particles of dark matter
decay into positrons.
There is a great deal more dark matter in the cosmos than the matter
we see as bright stars and galaxies. The evidence comes from the way galaxies
move, and from the inflationary theory of the origin of the Universe. Possible
candidates for this dark matterare very massive particles left over fromthe
David Eichler of Ben Gurion University in Israel has now calculated
the properties of one appropriate form of dark matter. Eichler set out to
explain the evidence of astronomers’ observations for an excess of positrons
in the cosmic ray background at energies above about 10 gigaelectronvolts.
Making standard assumptions about physics at very high energies (based on
Grand Unified Theories), Eichler showed that this positron ‘background’
would be produced naturally by the decay of very long-lived particles produced
in the big bang. Each would have a mass several thousand times that of a
proton (Physical Review Letters, vol 63, p 2440).
Eichler’s dark particles would now account for a density of about a
third ofa proton’s mass per cubic centimetre inthe inner part of our Galaxy.
In addition to producing positrons when they decay, the dark particles should
produce a background of gamma-rays with energies above 10 GeV.
The new generation of satellites may be able to observe these gamma-rays
in the 1990s. The satellites include the Gamma Ray Observatory Satellite
which is currently being planned. Such a detection would be the firmest
direct evidence that dark matter exists.