Add Yahoo as a preferred source to see more of our stories on Google. When you buy through links on our articles, Future and its syndication partners may earn a commission. The particles that are in ...

Add Yahoo as a preferred source to see more of our stories on Google. They're tiny, invisible, and travel across the universe. And trillions of them just flew through your body. What are they?

Every time I write about particle physics, I encounter a moment of uncertainty about a quantity that, at first glance, ought to be clear. How many kinds of elementary particles should I say there are?