Tornado Alley (PROG)

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F6, Invisible Galaxies: The Story of Dark Matter

.. just another good one i bumped into ..

By Matthew Craig and Sara Schultz

Introduction
Mysterious motion of the planets
Mysterious motions of galaxies
Invisible matter? Seriously?
Other ways to "see" dark matter
About the Authors
Activity 1: Dark Matter Possibilities
Activity 2: Dark Matter: Probing What You Can't See
Resources

Insert video:

http://www.youtube.com/watch?v=dipFMJckZOM

Introduction

Sometimes astronomers need a reality check. The other evening I was talking to my mother about dark matter, .. http://imagine.gsfc.nasa.gov/docs/science/know_l1/dark_matter.html .. and the more I said, the more she looked at me like I was nuts. It started when she asked me about my summer plans. The plans involve dark matter, so the question "What is dark matter? .. http://www.windows.ucar.edu/tour/link=/kids_space/dark_matter.html&edu=high .. arose almost immediately.

I explained that dark matter is this stuff that you cannot see, touch, taste, hear or feel, even with the most sensitive instruments. She looked suspicious. I added that most of the matter in the universe is dark matter. .. http://cdms.berkeley.edu/Education/DMpages/essays/ingredients.shtml .. She looked more suspicious. Getting a little desperate I confidently said the stuff is everywhere, even in our galaxy. Her face said she thought I was off my rocker. Out loud she asked, "How do you know this stuff really exists if you can't see it? Especially if there is so much of it!"?



In this image, dark matter (shown in blue) has become separated from luminous matter (shown in red) in the bullet cluster.

Image credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al

I realized then how poorly everyday life prepares one for understanding what the universe is made of. The Earth is a really exceptional place. Think about the kinds of things you encounter each day. You breathe air, which is mostly nitrogen and oxygen. You drink water, which is mostly oxygen by weight with some hydrogen. You eat food containing sugars composed of carbon, hydrogen and oxygen. The ground you walk on is composed of relatively heavy elements like silicon, iron and oxygen. It is natural enough to conclude the whole universe must be like this, a lot of heavy elements, a bit of hydrogen, and essentially no helium.

It is not.

The things you breathe, eat, and drink on Earth make up less than 1% of the sun. A star, like our sun, is about 75% hydrogen and 24% helium. Every object larger than a planet that we see in the universe is pretty similar to the sun in terms of its composition: mostly hydrogen, some helium, not much else.

So what about this dark matter stuff? Mom thought I was trying to change the subject so she gently reminded me to get to the point.

The point is that just as the materials commonly found on Earth don't represent what you observe in the sun, the materials that comprise the Sun don't represent all the matter there is in the universe. When you look at a galaxy the stuff that you see with a telescope is only a relatively small fraction of the stuff that is actually there. The stuff you can see—stars and gas—is made out of the same stuff as the sun; mostly hydrogen, some helium. The stuff you cannot see, called dark matter is...well, it is called dark matter. Nobody knows what it is.

About 80% of the matter in the universe is dark matter. .. http://map.gsfc.nasa.gov/m_uni/uni_101matter.html .. Describing a galaxy by saying it is made out of stars and gas isn't really accurate, in the same way that saying the sun is made out of helium isn't really accurate. Sure, there is some helium there, but most of the sun is not helium. Most of a galaxy is not stars and gas, it is dark matter.



Estimated distribution of dark matter and dark energy in the universe. Image credit: NASA

I was feeling pretty pleased with myself now. This seemed like a mighty fine explanation. Then Mom reminded me I still hadn't answered the big question: If no one has ever seen this stuff why do grownups (with Ph.D.s no less) believe this stuff exists?

The short answer is that the dark matter that we cannot see pulls, through gravity, on the matter that we can see. We "observe" the dark matter indirectly through the influence it has on ordinary matter.

You experience this sort of indirect detection in everyday life. As an example, imagine you are hanging a heavy picture on a wall. You need to put the nail into one of the wooden studs that supports the wall because the studs are strong and can support a lot of weight. Unless you have transparent walls you can't see the studs, so what do you do? One way to find a stud is to use a small magnet, moving it along the surface of the wall. When you pass over a screw attaching the wall to the stud, the magnet tries to stick to the spot, pulling on your hand a bit. You cannot see the screw but you can detect it by the pull it exerts on the magnet.

Mom would have been happier, I think, if I stopped there. But once she got me started there was no stopping. How can you not get excited about mystery stuff that permeates the universe? To understand in detail the evidence for dark matter a little history is useful.

.. much more .. http://www.astrosociety.org/education/publications/tnl/72/darkmatter.html