Please don't ask me another..!

One of the joys (?) of being a Grandad is that you are expected to have an answer for everything - and what's more, to be able to explain it in simple grandchild terms. So when my younger grandson said "So what's this dark matter stuff then?" I came out in somewhat of a cold sweat. The best I could come up with was this :- There's a known relationship between mass and gravitational force. We know that an object of a certain mass will produce a certain gravitational force. Look at that the other way round, and this means that if we detect a certain gravitational force, we can calculate the mass of the object which must be causing it. Scientists (don't ask me how) can calculate the total gravitational force in the Universe, and the fact is that there is not nearly enough observable mass to account for it. So there must be mass which we can't see or detect - it just has to be there to make the calculations add up. And because scientists are never happy unless they can label things, they've called this invisible mass "dark matter". And now I'm going home!