What is dark matter?

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  shavera
  Jul 20, 2011

  We have two major observational sources we base our knowledge of dark matter on. First, we realized that galaxies didn't rotate the way we'd expect if stars and gas and dust were the only matter within them. http://en.wikipedia.org/wiki/Galactic_rotation_curve Second, we can use the fact that mass causes light to bend to measure the amount of mass in a region. The most important observation here is the bullet cluster http://en.wikipedia.org/wiki/Bullet_cluster . Taken together, we realized that there had to be more mass out there in galaxies that doesn't reflect or emit light. (Since our earlier measurements were based on light measurements) Well that's okay, we know of particles that don't interact with light, they're neutrinos. But neutrinos are too lightly massed. They flit about the universe at very nearly the speed of light, so they aren't "cold" enough to settle around galaxies like we observe this missing mass to do. So we're looking for "cold dark matter."

  Right now, the leading candidate is a new family of particles that we may observe at the LHC in a few years. The idea is that all of our regular fundamental particles have supersymmetric "partners" that are really massive. The supersymmetric partners to the bosons (photons, W, Z bosons, gluons) have a particle called the neutralino. We expect that the neutralino will be the lowest mass supersymmetric partner particle we can find. And if we find it, we think that it may be the particle giving us cold dark matter. http://en.wikipedia.org/wiki/Neutralino

  Why didn't we know about it? Well it took the advent of modern astronomy. Great telescopes on earth and space to make really accurate measurements. We're building more telescopes and some earth-based experiments to try and suss out the details even more.