• Question: If dark matter supposedly exists in the universe, how come we can’t measure it exactly only estimate it? Therefore does it really exist?

    Asked by goddenl to Amy, Grant, Martin, Shawn, Usman on 13 Mar 2013.
    • Photo: Martin Archer

      Martin Archer answered on 13 Mar 2013:

      The reason why we think dark matter exists come from looking at how fast galaxies spin. Just like the planets orbit the Sun, the stars in a galaxy (like the Milky Way) orbit the galactic center. The speed they go around is given by the amount of mass contained within that orbit – so all stars/planets etc. closer to the center than that star. The problem is, the galaxies are spinning faster than they should do from all the matter that we can see. That’s why we think there must be some dark matter, something we can’t see, to account for this difference. We have some ideas of what it could be, but we’ve not been able to detect it because it must interact with normal matter very weakly – otherwise we’d have known about it for ages! Some people think it doesn’t even exist though and that our theories of gravity maybe break down over galactic scales and need changing.

    • Photo: Grant Kennedy

      Grant Kennedy answered on 13 Mar 2013:

      Just because we can’t measure something exactly doesn’t mean it doesn’t exist. What say I gave you a ruler and asked you to measure the distance to school. I bet you’d have a rough idea of the distance but wouldn’t actually get the right answer, because a ruler is a bad tool for the job. Of course, that doesn’t mean the road to school doesn’t exist, every measurement is really an estimate since it depends on how good your ruler is.

      What we see in galaxies is evidence for dark matter by looking at the way they spin, and as Martin explained, and it’s pretty convincing. So we actually get a pretty good measure of how much dark matter is needed in each galaxy.

      The problem is that there are other ways to get the same effect, by messing with Einstein’s theory of how gravity works. Because we can’t detect the dark matter directly, that means we don’t have absolute proof that it exists. So it’s not a problem of measurement, but one of not being able to tell the difference between two ways of getting the same spin effect in galaxies.
      g

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