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The Question

(Submitted October 21, 1997)

I am a real novice (advance apology for stupid question) but I first heard about dark matter last week and have been transfixed by this mystery. I basically understand (1) that particle physicists are trying to establish the existence of WIMP's in cryogenic crystals and in the ice shelf and (2) that part of the key to the dark matter theory is that as you move out from the center of a galaxy, the speed of the rotation stabilizes which implies that there is mass there that does not emit light. My question is do particle physicists believe that WIMP's are dispersed throughout the galaxy? And if so, why wouldn't WIMP's affect the orbit of other bodies within the galaxy (i.e. our own solar system)?

The Answer

Two likely possibilities for the dark matter in our own galaxies are MACHOs (MAssive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). MACHOs are low mass stars, brown dwarfs, neutron stars and white dwarfs. If MACHOs make up most of the dark matter, the distribution is not smooth on the scale of the Solar system, but it is smooth on a much larger scale.

If the Galactic dark matter consists of WIMPs, then they are dispersed throughout the Galaxy, with a distribution somewhat different from that of the stars that we can see. Since gravitational pulls of WIMPs from different directions tend to cancel out, the orbit of planets in our solar system is not affected by the presence of WIMPs. However, since there are more WIMPs towards the center of our Galaxy than away from it, the motion of the Solar system (and other stars) in the Galaxy is strongly affected --- this is how astrophysicists infer the presence of the dark matter.

Best wishes,

Koji Mukai
for Ask an Astrophysicist

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