The quest for understanding the nature and amount
of dark matter is one of the most important investigations that NASA can
undertake.
The amount of dark matter is fundamental to establishing
whether the universe will continue to expand indefinitely.
The nature of dark matter (baryonic vs. non-baryonic)
could profoundly change our view of the physical world.
What is Dark Matter?
We generally learn about the universe based on the 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 doesn't glow? How do we know it is there? How do we
know how much is there? How do we know what it is?
Most of the universe is "dark matter", that is, material
that does not emit sufficient light for us to directly detect it. There
are a variety of ways that allow us to indirectly detect this material.
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 the extra gravity that we detect we infer how much mass must be present.
The kinds of material that we experience every day is made of atoms,
which are, in turn, made of proton, neutrons, and electrons. We refer to
this type of material as "baryonic". Is the dark matter in the
universe made of the same stuff that we are familiar with on Earth, i.e.,
baryonic? Or is it some strange and exotic new material, i.e., nonbaryonic?
The answer appears to be that the universe is comprised of both kinds of
dark matter. In fact, it is likely that most of the material in the universe
is of some mysterious form that we can not yet identify!
Efforts are under way to identify the nature of both types of dark
matter. Proposed space missions, when carried out, will shed light on the
dark matter. Studies of the cosmic microwave background (the 3 degree thermal
afterglow radiation from the Big Bang) can reveal how much material there
is, how much is baryonic and how much is nonbaryonic, and the nature of
the dark matter. X-rays satellites will probe the gravity around galaxies,
galaxy groups and galaxy clusters to infer the total amount of dark matter.
They will use possible absorption of the cosmic X-ray background to detect
(or set limits) on the presence of a halo component of dark "baryonic"
matter. By opening these new windows with new space missions, humans will
know, for the first time, what our Universe is made of.
