In 1967, Vera Rubin observed that stars
within the Andromeda galaxy had
higher-than-expected orbital speeds.
Physicists have also observed the same
phenomenon in the nearby Triangulum galaxy.
By measuring the orbital speeds of stars within
Triangulum and using the formula
physicists have calculated that the mass of this
galaxy within a radius of r = 4.0 Ã 1020 m
is equivalent to 46 billion Suns.
However, by measuring the brightness of
Triangulum, they have also calculated that its
mass within a radius of r = 4.0 Ã 1020 m is
equivalent to 7 billion Suns.
The discrepancy between these two results
implies that there is 39 billion Suns' of unseen
mass within Triangulum.
This unseen mass is called "dark matter".
Physicists have observed many other galaxies
and most are now convinced that, on average,
dark matter accounts for 90% of the mass of
every single galaxy in the universe.
Physicists also have independent evidence for
the existence of dark matter from observations
of distorted images of distant galaxies
(gravitational lensing).
Although no one knows what dark matter is
made of, physicists currently have a number
of theories.
One of the earliest theories of dark matter was
that it consists entirely of compact celestial
objects such as planets, dwarf stars, and
blackholes. Careful observations have ruled
out this theory.
Most physicists today think that dark matter is
made of a type of subatomic particle that, to
date, has never been detected in the laboratory.
The two leading candidates are weakly
interacting massive particles (WIMPs)
and axions.
Numerous experiments that are trying to
detect one of these particles are currently
underway worldwide.
As physicists do not yet know what dark matter
is made of, they do not know the composition
of a large fraction of the universe.
