Dark matter and the matter of Trader Joe's advertising

I've been thinking about 'dark matter' a lot lately. Dark matter, for those of you in the, uh, dark, is undetectable, invisible stuff that so far exists only in theory. If it's undetectable, you may think, there's no reason to even think about it. But there is if you're an astronomer, because when you look out at the universe, the known laws of physics (mainly gravity, in this case) can't explain the rate at which the universe appears to be expanding.

To explain the current rate of expansion, the universe basically has to weigh far, far more than the weight of all the stuff in it that we can detect. So, for the observed universe to make sense to physicists and astronomers, they now all agree that most of the universe's mass is made up of something they're calling dark matter.

I'm not usually preoccupied with astrophysics, but lately I've taken a bit of heat for the subtitle of my book. A comment that I get a lot is, "But Trader Joe's does advertise! I get their flyers," or "I hear Trader Joe's radio ads," or "Isn't their web site a kind of ad?"

Yes, you're right. So am I. To learn why, click 'Read more' below.