New ScienceBlogger and American Gladiator Ethan Siegel of Starts With a Bang has a couple of nice posts about dark matter and how we know it's there (one, two).
These posts reminded me that I never did follow up on the discussion following my post about Magic World Media, who are looking to publish kids' books about science, and seeking kindergarten-level explanations about dark matter and dark energy. A few commenters expressed doubts about whether this could or even should be explained to kids at that level.
I don't have any strong feelings about the "should" question, but it seems to me that there's no reason why you couldn't construct an explanation of how we know where there's dark matter. I say this mostly because I'm currently a week and a bit into an intro mechanics course using the Matter and Interactions curriculum.
I'm not entirely sure what would constitute a kindergarten level explanation, but the first chapter of M&l;I is a very qualitative discussion of interactions and how to detect them, that seems like it could easily be adapted to a really young audience. The basic point they make (at some length) is that you can detect interactions between objects by looking for changes in motion. If you see one object moving along, and it changes direction, that tells you that it interacted with something else. If you see one object moving along, and it changes speed, that tells you that it interacted with something else.
You could easily pitch this to a young audience by showing pictures of single objects moving and changing their motion, but not what they interact with. Then you could follow with a picture that zooms out or removes an obstacle to show the other interacting object. (For example, you show a ball rolling down the street picking up speed, and then pull back to show that it's rolling down a hill. Or you show a dog walking along the street and then veering off in some direction, and then shift perspective to show an angry dog hidden behind a fence, or a bunny to be chased.)That gets you the idea that even if you can't see what an object is interacting with, the fact that its motion changes tells you that there's an interaction taking place.
And that is, essentially, how we know that dark matter exists. When we look at other galaxies, and how the stars in those galaxies move, or how whole clusters of galaxies move, we see their motion changing in ways that we can't explain through interactions with things that we can see. That tells us that there has to be someother interaction taking place, with objects that we can't see.
So, I don't think it's impossible to come up with an explanation of how we know that dark matter exists that would work for young kids. It wouldn't be at all quantitative, but on a conceptual level, I think you could get the idea across. And given the right sorts of illustrations, it could be both charming and effective.
Whether you really need to teach kids that young about dark matter is another question, but I don't see any reason why it couldn't be done.
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Since I commented that it would be difficult to explain, I will speak up here to say this is a good explanation that my first grade son would understand. I'm not sure if dark energy is as simple, but maybe someone that understands it better than I do explain it on a similar level. I still think there are a bunch of simpler and more relevant concepts I would try to tackle first.
While I was finishing up my undergrad I had a whole pile of ideas for explaining some of these concepts to kids very simply, but never did anything with it.
Do you need to teach little kids about dark matter or electromagnetism? No. Do you need to teach them about dinosaurs or Mars or ancient Egypt or the bottom of the sea? Again, no. But these are all cool and interesting things, so if kids are interested in them, why not? (I'm not saying you need to add them to the kindergarten curriculum, mind you, but I'm all for having books available at the appropriate reading and comprehension level for them as wants them.)