271
cannot directly be observed; only
its effects are detectable, and the
only effects that can be detected
are from its gravity. It does not
interact with the electromagnetic
force, meaning that it does not
absorb heat, light, or other radiation,
nor does it emit any. Dark matter
may be completely invisible.
Possible sources
The simplest solution to the dark
matter problem is the most literal.
It comprises ultra-dense bodies of
ordinary matter that are too dark to
observe. Astronomers have dubbed
these MACHOs, which stands for
Massive Compact Halo Objects.
MACHOs include objects like black
holes, neutron stars, and white and
brown dwarfs. They occupy the
galactic halo, a dark and diffuse
region that surrounds the main,
shining disk of a galaxy—and this
is why it is difficult to see them.
MACHOs are clearly out there, but
by current estimates they would
only account for a tiny proportion
of dark matter. An alternative idea
THE TRIUMPH OF TECHNOLOGY
is the WIMP—Weakly Interacting
Massive Particle. This concept is
based largely on an idea in particle
physics called supersymmetry.
It proposes a new explanation of
energy and ordinary matter. Energy
and matter form two distinct
groups of subatomic particles,
and supersymmetry proposes that
these groups interact thanks to the
actions of “super particles,” or
sparticles. Dark matter WIMPs
might be sparticles that escaped
from their partners in the early
period of the universe, or they may
be objects that are there all along.
Finally, dark matter might be
the observable effect of another
universe, or perhaps several,
that exist in a spatial dimension
different from this universe. Their
matter could be very close, a few
centimeters away, but because
the radiation from each universe is
trapped inside its own spacetime,
one universe can never see another.However, the gravitational effects
of the matter in the hidden universes
leaks through into this one through
the warping of spacetime.
Providing an explanation for dark
matter remains one of the biggest
prizes in astronomy. However, in
1999, a possibly even more puzzling
phenomenon was uncovered. It was
discovered that 68 percent of the
universe was neither matter nor
dark matter, but so-called dark
energy. Dark matter makes up 27
percent; visible matter comprises
a mere 5 percent. ■A huge ring of dark matter, which
formed long ago in the collision between
two massive galaxy clusters, is shown
around the edge of this Hubble Space
Telescope image in lighter blue.Dark matter could be evidence that
the universe is one of many that exist
next to one another, in separate spatial
dimensions, in a bubblelike multiverse.
For the moment we
might very well call them
DUNNOS (for Dark Unknown
Nonreflective Nondetectable
Objects Somewhere).
Bill Bryson