The nature of dark matter
In general, scientists learn about the Universe by the electromagnetic radiation (or light) that we can observe emanating
from the objects in it. The light we see is in the form of radio waves,
optical, ultraviolet, X-ray, and gamma-ray emission. But what if there is material
in the Universe that does not glow in a way that we can directly
observe? 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?
Back in 1933, astronomer Fritz Zwicky was studying the Coma
Cluster of galaxies, and he noticed that the amount of matter needed
to result in the orbital speed of the galaxies did not reflect the
amount of matter that was visually detectable. Many more observations
with similar characteristics have been made since then, and scientists
have used these observations to determine that the gravitational
potential required to explain the observations implies a significant
amount of what Zwicky termed "missing mass" – something we now
commonly refer to as "dark matter."
Coma Cluster – The image above
is composed of multiple superimposed images: the red part of the
image is an Einstein satellite X-ray image, and the blue
image is a Palomar Sky Survey optical image.
Astronomers now think that most of the material in the Universe is
made of dark matter. It is matter that does not emit sufficient light
for us to directly detect it. However, there are a variety of ways that
we can indirectly detect it. The most common method involves the fact
that the dark
matter, like other forms of visible matter, has a gravitational
influence on the light and 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 kinds of materials that we experience every day are made of
atoms, which are made of protons, neutrons, and electrons. We refer to this type of matter as "baryonic." The foremost question
that scientists are working to address is whether dark matter is made of
the same stuff that we are familiar with, i.e., is it baryonic, or is it
some kind of exotic new material? So far, it appears that there is both
baryonic and non-baryonic dark matter, and the Universe appears to be
composed of both kinds. The scientific realm is still very active in the
effort to discover the nature of each variety.
Updated: June 2011