Question: what is dark matter?

Sara Imari Walker answered on 15 Jun 2011:

We don’t know what dark matter is, but we do have some clues. The term dark matter arose because it appears that most galaxies have extra mass that doesn’t interact electromagnetically (i.e. we can’t see it since it doesn’t emit light like stars). The existence of this dark matter was first surmised by looking at galatic rotation curves – as you move away from the center of galaxies you should see the orbital velocity (their speed circling the galatic center) of stars slowing down as the density of stars drops off due to less gravitational attraction. Instead you observe a constant oribital speed – even to the far edges of galaxies -as you move away from the galatic center -indicating that there is some unseen matter there that interacts gravitationally! So we don’t know what dark matter is but we know it interacts via gravity. We do have a few candidates for dark matter though. One is brown dwarfs – those are stars that didn’t quite make it to Hydrogen burning so they are effectively dark dead stars. Another is small black holes. My personal favorite is WIMPS (weakly interacting massive particles) a hypothesis that claims there are new fundamental particles that we haven’t detected yet (fundamental being something like an electron or a quark). WIMPS would have to have some very special properties to be dark matter – imagine a particle that doesn’t interact with light! I did a project once on axions – those are pretty cool little particles – I hope they exist!! But there are ALL kinds of particle candidates ranging from supersymmetric (like a mirror particle with large mass and some opposite properties for all known particles) of even sterile neutrinos (these are really heavy neutrinos). Pretty wild!!