surprise, his results suggested
that the cluster contained about
400 times more mass than that
suggested by the combined light
of its stars. Zwicky called this
staggering amount of unseen
matter “dark matter.”
Zwicky’s conclusion was largely
overlooked at the time, but by the
1950s, new technology had opened
up new means of detecting
nonluminous material. It was clear
that large amounts of matter are
too cool to glow in visible light but
still radiate in infrared and radio
wavelengths. As scientists began
to understand the visible and
invisible structure of our galaxy
and others, the amount of “missing
mass” fell substantially.
The invisible is real
The reality of dark matter was
finally recognized in the 1970s,
after US astronomer Vera Rubin
mapped the velocity of stars orbiting
in the Milky Way and measured
the distribution of its mass. She
showed that large amounts of
mass are distributed beyond the
galaxy’s visible confines, in a
region known as the galactic halo.
Today it is widely accepted that
dark matter constitutes around
84.5 percent of the mass in the
universe. Any hopes that it might
actually be normal matter in hard-
to-detect forms, such as black holes
or rogue planets, have not been
borne out by research. It is now
thought that dark matter comprises
so-called Weakly Interacting
Massive Particles (WIMPs). The
properties of these hypothetical
subatomic particles are still
unknown—they are not only dark
and transparent, but they do not
interact with normal matter or
radiation except through gravity.
Since the late 1990s, it has
become clear that even dark
matter is dwarfed by “dark energy.”
This phenomenon is the force
accelerating the expansion of the
universe (pp.236–41), and its nature
is still unknown—it may be an
integral feature of space-time itself,
or a fifth fundamental force known
as “quintessence.” Dark energy is
thought to account for 68.3 percent
of all the energy in the universe,
with the energy of dark matter
amounting to 26.8 percent, and
normal matter a mere 4.9 percent. ■A PARADIGM SHIFT 251
See also: Edwin Hubble 236–41 ■ Georges Lemaître 242–45
Fritz Zwicky
Born in Varna, Bulgaria, in
1898, Fritz Zwicky was raised
by his Swiss grandparents
and showed an early talent for
physics. In 1925, he left for the
US to work at the California
Institute of Technology
(Caltech), where he spent
the rest of his career.
Aside from his work on
dark matter, Zwicky is also
known for his research into
massive exploding stars. He
and Walter Baade were the
first to show the existence of
neutron stars intermediate in
size between white dwarfs
and black holes, and coined
the term “supernovae” for the
enormous stellar explosions
in which these massive
stellar remnants are born.
By showing that one class
of supernovae always reach
the same peak brightness
during their explosions, they
also provided a means of
measuring the distance to
far-off galaxies independently
of Hubble’s Law, paving the
way for the later discovery
of dark energy.Key works1934 On Supernovae
(with Walter Baade)
1957 Morphological AstronomyIf our galaxy’s mass distribution matched that of its visible matter,
then stars in the galaxy’s outer disk would move more slowly at greater
distances from the massive center. Vera Rubin’s research found that beyond
a certain distance the stars tend to move at a uniform speed regardless of
their distance from the hub, revealing dark matter in the galaxy’s outer halo.
Rotational velocity
[km/s]Distance from center of galaxy (light years)20050,000 100,000MeasuredCalculated100