The Nature of Dark Matter
In general, scientists 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 the dark matter has a gravitational pull 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". 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 is both a kind of baryonic and non-baryonic dark
matter, and it appears to be that the Universe is composed of both kinds.
In fact, it is likely that most of the matter in the Universe is of
some mysterious form that we cannot yet even identify!
The image at the left is of the Coma Cluster. It consists of two
superimposed images: the red image is an Einstein satellite X-ray
image; the blue image is a Palomar Sky Survey optical image. Scientists
have used these observations to determine that the gravitational
potential required to explain the observations implies a significant
amount of "missing mass."