Particle identity of dark matter

4 problems

• Is dark matter the QCD axion?

  Open

  The axion is a very light hypothetical particle invented to explain why the strong force does not distinguish past from future in a certain way, and it could also be dark matter. Experiments try to convert axions into microwave photons inside strong magnets.

• Is dark matter self-interacting with a velocity-dependent cross section?

  Open

  If dark matter particles collide with each other, the centers of halos are smoothed into cores, and some cores later collapse into very dense centers. The collision strength may depend on speed, being larger in small galaxies than in clusters.

• What is dark matter made of?

  Open

  Galaxies, clusters and the cosmic microwave background all require extra unseen mass, about five times the ordinary matter. The task is to find out what that mass physically is.

• Is dark matter a thermal-relic weakly interacting massive particle?

  Open

  A popular idea is that dark matter is a heavy particle that was in equilibrium with ordinary matter in the hot early universe and froze out at the right abundance. Underground detectors look for it hitting atomic nuclei.