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Can anyone give a brief overview of what a wimpy particle is?

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up vote 5 down vote accepted

A little history because I don't like the formulation of the question title.

Galaxies exhibit a rotational behavior that is inconsistent with the known law of gravitation and the visibly luminous mass distribution of the galaxy. Two ideas present themselves to explain why:

  1. There is some non-visible mass
  2. Gravity doesn't work the way we thought

Well, option 2 is pretty heady stuff, so lets look first at how option 1 would work. Maybe the so-called "missing mass" is just cold. That way it wouldn't glow and we wouldn't see it. Well, infrared and radio astronomy would see it if it was spread out in a diffuse gas, and they see a little, but not nearly enough.

So, maybe the missing matter is in big cold lumps. Call them MAssive Compact Halo Objects (MACHOs). Astronomers did extensive microlensing searches for MACHOs in the Milky Way during the 90s, and they found a few but can rule out most scenarios in which MACHOs contribute a large fraction of the missing mass.

That leaves us looking for some mass that simply doesn't glow at any band. That is, we're looking for matter which does not interact with the electromagnetic force, but does interact gravitationally. If we make the conjecture that it also interacts by way of the weak force we can call this stuff Weakly Interacting Massive Particles (WIMPs), which makes a nice counterpart to MACHOs.

The "wimpy" in the title is just the adjectival form of WIMP.

Computer models of cosmological evolution that assume the dark matter has the properties expected of WIMPs can explain the combined lumpiness and large scale uniformity of the universe, so there is considerable hope that this is the explanation we're looking for.

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