I think it's not an overstatement to say that it's quite a challenge to explain dark matter to elementary school kids. To convey to kids the idea that something like 95% of the universe is this mysterious dark matter and dark energy, I used this:
That's right. The stuff that we can detect--you and I and the world, the stars and planets in the universe--are represented by the colored jelly beans. All the rest? Licorice jelly beans.
The guy at Art Mart was very helpful when I told him I wanted to buy a mixture that was 95% licorice and 5% colored. I told him it was a teaching aid and commented that it must be an unusual request.
"Oh, you'd be surprised," he said.