• Question: Is dark matter made from photons, neutrons and electrons or with something else?

    Asked by anonymous to Steve, Emma, Christian, Bose, Alice on 9 Mar 2016.
    • Photo: Alice Harpole

      Alice Harpole answered on 9 Mar 2016:

      We don’t really know! Currently, it’s thought that dark matter is made up of ‘weakly interacting massive particles’ (WIMPs) – in the past it was thought it could also be made up of ‘massive astrophysical compact halo objects’ (MACHOs) such as neutron stars or black holes, but there is now lots of evidence to say this is probably not the case. (You may have noticed that physicists can sometimes have a lot of fun when naming things!)

      WIMPs are particles that interact only through gravity and the weak force (the fundamental force responsible for processes like radioactive decay). These forces are both very weak compared to the other fundamental forces (electromagnetism and the strong force), hence why it’s called dark matter – we don’t see it as it doesn’t interact much. There’s some evidence that WIMPs are not baryons (protons and neutrons) or neutrinos. The universe is thought to have neutral charge overall, so WIMPs cannot be charged particles like electrons. If they were photons, we’d see them as electromagnetic radiation. This leaves something called ‘non-baryonic matter’ – hypothetical particles like axions or supersymmetric particles that we are yet to detect experimentally.

    • Photo: Steve Marsden

      Steve Marsden answered on 9 Mar 2016:

      It’s definitely not photons, protons, neutrons, electrons or neutrinos… but that’s about all we know about it. We know that the particles that make it up must be over a certain mass by the shape they form around the galaxy. We know that they must not interact with photons, otherwise they wouldn’t be dark and we’d have seen them.

      As Alice said, the current best theory is that they are WIMPs. We’re looking for these particles using two methods.

      Firstly we’re trying to detect them directly. Most of the time if a WIMP passes through something, it will do so without interacting. It will stream straight through and out the other side. In very, very rare situations though, it may collide with an atom inside the object. To detect them, giant tanks are filled with fluids that cause a flash of light if a WIMP interacts with it. This fluid is then watched continuously, waiting for that signal.

      Secondly, we’re trying to make more dark matter. I work on the LHC, where we collide huge amounts of energy together. We know that the energy can turn into new particles. Most of the time, the new particles created are things we already know and understand. It’s possible however, that some of the energy may be turned into dark matter. We wouldn’t be able to store or measure this dark matter, but we’d notice that some of the energy is missing. This would then start to give us a handle on the properties of the new matter.

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