The Nature of Dark Matter
Cluster of galaxies
In general, astronomers learn about the Universe by the electromagnetic
radiation (or light) that we see from it. The light we see is in the form of
radio waves, infrared, optical, ultraviolet, X-ray, and gamma-ray emission.
But what if there is material in the Universe that does not glow? How will we
ever know it is there? How can we tell how much of it there is? How do we
know what it is?
Such material is called "dark matter", and astronomers now believe that most
of the material in the Universe is made of this stuff. It is material that
does not emit sufficient light for us to directly detect it, yet there are a
variety of ways that we can indirectly detect it. The most common method
involves the fact that dark matter has a gravitational
pull on both the light and the sources of light that we can see. From the effects of "extra" gravity
that we detect, we infer how much mass must be present.
The image above shows one way this is done. Pictured here are two
superimposed images of the Coma Cluster of galaxies. The red areas are
X-ray light seen by the Einstein satellite; the blue is visible light
from a Palomar Sky Survey optical image (made with ground-based
telescopes at Caltech). Scientists have used these observations and
others to determine the
amount of gravity required to hold together all the mass detected in the
image. Surprisingly, there is not nearly enough mass observed to explain the
inferred gravity - somehow, there is undetected "missing mass."
What could this "missing mass" be?
The kinds of materials that we experience every day are made of atoms, which
are composed of protons, neutrons, and electrons. We refer to this type of matter
as "baryonic". Is the dark matter in the Universe made of the same stuff that
we are familiar with, i.e., is it baryonic? Or is it something strange ... some
kind of exotic new material, which we could call non-baryonic?
So far, it looks like there are both baryonic and non-baryonic types of dark
matter. Some dark matter may be composed of regular matter (ie.,
baryonic), but simply not give off much light. Things like brown dwarf
stars would be in this catagory. Other non-baryonic dark matter may be tiny,
sub-atomic particles which aren't a part of "normal" matter at all. If
these tiny particles have mass and are numerous, they could make up a
large part of the dark matter we think exists. If true, then it's
possible that most of the matter in the Universe is of
some mysterious form that we cannot yet even identify!