• Question: Whats the meaning of dark matter?

    Asked by leahrogers00 to Christina, Colin, Jess, Samaneh, Steve on 17 Jun 2013.
    • Photo: Steven Gardner

      Steven Gardner answered on 17 Jun 2013:

      Dark matter is stuff we know should be in the universe, but can’t find. We can measure the effect it has on the stuff we can see (stars and galaxies), but because it emits no light itself we can’t gain any information about it.

      Now you might say, “well there must be loads of planets and asteroids and comets we can’t see from this far away, why can’t that be dark matter?”. The answer to that is all those things combined won’t be anywhere near enough, more than 99% of the mass of any star and planet system will be contained within the star (The sun is 300,000 heavier than the Earth!), so counting all the stars should be enough, but when we do that the final number isn’t big enough, in fact it’s no where near what it should be. Therefore there must be other matter than is there but we can’t see it. We call this dark matter.

    • Photo: Christina Pagel

      Christina Pagel answered on 17 Jun 2013:

      Hi Leah,

      Ooh that’s a tricky question… ok here goes:

      Outside of our planet, we can only learn about how things work by looking at them – and that means they have to somehow let us know they’re there! The things we can see directly, like stars and galaxies, let us know they’re there by emitting light (both visible light and other types of light like x-rays or radio waves) so we can see them with telescopes. We can also see how they move – and all stars and galaxies are moving, and more to the point, everything is generally moving away from everything else! Because we can see them and measure how fast they’re moving compared to everything else, we know that they are not moving how they ‘should’ be if what we can see is all there is. In fact, in our current version of the laws of physics, for the things we can see to moving as they are, means that there *must* exist lots of stuff we can’t see. And because we can’t see it, it’s called “dark matter”. It’s not called anything more precise because no one really knows what it is!! Scientists are doing more and more experiments to try to work out what it is, how it got there, how it relates to the things we can see, but as of now noone really knows!

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