From: "matt k"
Date: Monday, November 08, 2004 6:09 PM

Q You say that dark matter is a cousin particle to our normal periodic table, does that mean that it is possible that there are dark people? I  realize that you say that dark matter has different properties. All im wondering is if it is possible. Thanks for taking the time to answer my email.

A We can’t really be sure, since we haven’t yet figured out for certain what the dark matter even is, but physicists don’t generally think that there are people made solely of dark matter. The basic reason for this is what you mention – the different properties of ordinary matter and dark matter (or, at least, WIMP dark matter - our best guess as to what the dark matter is made up of).

In order to make a person (or a planet, or any other ordinary object), you need to gather a bunch of particles into one region and get them to stick together.  In your body, the electromagnetic force (attraction between positive and negative charges) makes atoms stick together to make chemicals and complicated structures. This same force holds the atom itself together – the positively-charged nucleus is attracted to the negatively-charged electrons. The nucleus in the center of the atom is made of protons and neutrons, held together by the strong nuclear force.

WIMPs (the particles believed to make up the dark matter) don’t “feel” the electromagnetic or strong nuclear forces.  This means that they can’t be held together to form things like atoms (or people).  If you made a pile of WIMPs the size of a person, there would be nothing to hold it together and it would just fall apart.  WIMPs do feel the weak nuclear force, but that’s not very useful for holding things together (it is, however, how the CDMS experiment hopes to detect them!). Most importantly they also feel gravitational forces, which allow dark matter to affect the motions of stars and planets.  This allows dark matter particles to be held together in big, loose clouds the size of entire galaxies, but isn’t strong enough to make small, solid objects (like people).

Hope that helps!

Jeff Filippini
Berkeley Cosmology Group