>Date: Thu, 24 May 2001 09:23:15 -0700
>Subject: Ouestion on dark matter!
>
>Hi I am a senior in High School and I am doing a report on dark 
>matter for a sicence class... I have read many many articles on "dark 
>matter" but I have one question I am not sure what the difference is 
>between the "wimps" and the "machos"
>thank you,
>
>Fallon


	So, there have been at least two very successful theories about the
nature of dark matter. Some have suggested that dark matter is not made of some
strange new particle but consists of ordinary but hard-to-see failed or dead 
stars or even huge cold planetary objects.  Since this type of dark matter is 
not in the form of particles strewn through space, but rather is in the form of
clumpy objects we call these hypothesized masses "compact".  These dark matter 
objects would be accumulated inside the galaxies, particularly about the center 
of each galaxy.  We call this cluster of dark matter about the center of each 
galaxy a "halo".  Thus, these hypothesized objects would be massive, compact
and they should be found in the halo of the galaxy. So, they are called
"Massive Compact Halo Objects" or "MACHOs" for short.

	WIMPS are the other very popular candidate for dark matter.
Unlike MACHOs, they aren't in the form of clumpy objects gathered in
galactic centers but are undiscovered particles which were created in the
Big Bang and exist everywhere.  However, their numbers are greater in the
centers of galaxies and they would exist in the form of halos joined to most
every galaxy.  We know that if these particles exist, they interact with 
ordinary matter very weakly.  And, even though each individual hypothesized 
particle is probably very light, they are probably still massive compared to 
other particles with which we are familiar (perhaps 100 times heavier than
a hydrogen atom)  So, we call them "weakly interacting massive particles" or 
"WIMPs" for short.

	Now, from a variety of studies like gravitational lensing observations,
we know that do MACHOs exist.  But, they probably don't account for much
more than a few percent of the missing mass.  We are guessing that the
rest is WIMPS.  Our experiment may tell us more within a year or two!...

Michael Scott Armel
CfPA