From: "Kamel"
Date: Mon, 07 Feb 2005 09:28:07 -0800

Q I have question related to the origin of dark matter and galaxies. My question is what causes this dark matter? Is this dark matter located near the center of our galaxies? thank you for your time.

A Excellent question, Kamel. In fact, it's such a good question that some of the brightest people in the world are still trying to figure out the answer. So far, we're still not sure exactly what the dark matter really is. There is a lot of evidence that dark matter exists, and that there is a lot more dark matter than visible matter (stars, gas, etc.). There is also evidence that tells us that nearly all of this dark matter is "non-baryonic" - in other words, it's not made up of protons, neutrons, and electrons the way ordinary matter is. The current best guess of most cosmologists is that dark matter is made up of new kind of particle, called a weakly-interacting massive particle (WIMP), produced in the Big Bang. We think these particles are all around us, not just in the galactic center - in fact, billions of these particles might pass through your body every second. WIMPs are thought to form a big cloud (called a "dark matter halo"), much larger than the visible part of the galaxy. Our galaxy would be immersed in this cloud, sitting near the center. The cloud is probably densest at the center, so we expect there to be more WIMPs near the galactic center than in the outskirts of the cloud, but dark matter should be everywhere and not just at the galactic center. A similar cloud would surround each galaxy in the universe, and bigger "super-clouds" would surround entire clusters of galaxies. Of course, we are not yet sure that this is right. Many groups of scientists are currently looking for these particles, either by creating them in particle accelerators or detecting them in the world around us. Cosmologists are also busy trying to think up other good explanations for dark matter. Hopefully we'll find out more in the upcoming years.

Hope that helps, and let us know if you have any more questions.

- Jeff Filippini
UC Berkeley Cosmology Group